Multilayer piezoelectric element and injector using the same

ABSTRACT

In a multilayer piezoelectric element in which a plurality of piezoelectric layers and a plurality of metal layers are stacked alternately, the plurality of metal layers include a plurality of low-filled metal layers having a lower filling rate of metal composing the metal layers than oppositely disposed metal layers adjacent to each other in a stacking direction. In a multilayer piezoelectric element in which a plurality of piezoelectric layers and a plurality of metal layers are stacked alternately, the plurality of metal layers include a plurality of thin metal layers having a smaller thickness than oppositely disposed metal layers adjacent to each other in a stacking direction. In a multilayer piezoelectric element in which a plurality of piezoelectric layers and a plurality of metal layers composed mainly of an alloy are stacked alternately, the plurality of metal layers include a plurality of high-ratio metal layers having a higher ratio of a component constituting the alloy than oppositely disposed metal layers adjacent to each other in a stacking direction.

TECHNICAL FIELD

The present invention relates to a multilayer piezoelectric element (insome cases hereinafter referred to simply as an “element”) and aninjector, and in particular, to a multilayer piezoelectric element andan injector which are suitable for a long-term continuous driving underhigh voltage and high pressure.

BACKGROUND ART

As an example employing a multilayer piezoelectric element,piezoelectric actuators in which piezoelectric layers and metal layersare alternately stacked one upon another have conventionally beenproposed. In general, the piezoelectric actuator can be classified intothe following two types of simultaneous sintering type and multilayertype in which piezoelectric porcelains consisting of a piezoelectricbody and metal layers of plate-like body are alternately stacked oneupon another. Among others, the simultaneous sintering typepiezoelectric actuators are often used from the viewpoints of lowervoltage and manufacturing cost reduction. The simultaneous sinteringtype piezoelectric actuators facilitate a reduction in layer thicknessand have excellent miniaturization and durability.

FIG. 21( a) is a perspective view showing a conventional multilayerpiezoelectric element. FIG. 21( b) is a partial perspective view showingthe stacked state of piezoelectric layers and metal layers in FIG. 21(a). FIGS. 22 and 23 are partially enlarged cross sections showing thestacked structure in the conventional multilayer piezoelectric element.As shown in FIG. 21, the multilayer piezoelectric element is composed ofa stacked body 103, and a pair of external electrodes 105 formed onopposed side surfaces, respectively. The stacked body 103 is configuredby alternately stacking piezoelectric layers 101 and metal layers 102.Inactive layers 104 are stacked on both end surfaces of the stacked body103 in the stacking direction, respectively. The metal layers 102 arenot formed entirely over the main surfaces of the piezoelectric layers101, thereby forming a so-called partial electrode structure. The metallayers 102 in the partial electrode structure are stacked so as to beexposed by every other layer to different side surfaces of the stackedbody 103, and the metal layers 102 are connected by every other layer tothe pair of external electrodes 105.

A conventional method of manufacturing the conventional multilayerpiezoelectric element is as follows. That is, firstly, a metal paste isprinted on a ceramic green sheet containing the raw material of thepiezoelectric layers 101, in such a pattern as shown in FIG. 21( b),which forms a predetermined metal layer structure. Then, a plurality ofthe green sheets with the metal paste printed thereon are stacked oneupon another to prepare a stacked forming body. The stacked forming bodyis then sintered to obtain the stacked body 103. Thereafter, the metalpaste is applied to the opposed side surfaces of the stacked body 103,and then sintered to form a pair of the external electrodes 105,resulting in the multilayer piezoelectric element as shown in FIG. 21(a) (for example, refer to Patent Document No. 1).

As the metal layers 102, in general, an alloy of silver and palladium isoften used. In order to simultaneously sinter the piezoelectric layers101 and the metal layers 102, the metal composition of the metal layers102 is often set to a 70% by mass of silver and a 30% by mass ofpalladium (for example, refer to Patent Document No. 2). The followingis the reason that the metal layers 102 composed of the alloy of silverand palladium are used instead of the metal layers consisting only ofsilver.

That is, the composition of the metal layers 102, which consists only ofsilver and contains no palladium, causes so-called ion migrationphenomenon that when a potential difference is applied to between theopposed metal layers 102, the silver ions in the metal layers 102migrate through the element surface, from the positive electrode to thenegative electrode in the opposed metal layers 102. This phenomenontends to occur remarkably in the atmosphere of high temperature and highmoisture.

On the other hand, for the purpose of forming the metal layers 102 ofsubstantially identical metal filling rate (proportion), a metal pastewhose metal composition rate and metal concentration are prepared so asto be substantially the same has conventionally been used. When thismetal paste is screen-printed on the ceramic green sheet, the stackedbody 103 is prepared by setting a mesh density and a resist thickness tosubstantially the same condition. In the metal layers 102 formed withthis metal paste, voids 102′ can be formed nearly uniformly, as shown inFIG. 22.

As shown in FIG. 23, for the purpose of forming the metal layers 102 ofsubstantially identical thickness, a metal paste whose metal compositionrate and metal concentration are prepared to be substantially the samehas been conventionally used. When this metal paste is screen-printed onthe ceramic green sheet, the stacked body 103 is prepared by setting amesh density and a resist thickness to substantially the same.

In the case of pressing and stacking ceramic green sheets, the metallayers 102 have a partial electrode structure. Therefore, the area wherethe metal layers 102 are overlapped with each other, and the area wherethe metal layers 102 are not overlapped with each other have differentpressed states. As a result, the metal layer density may becomenon-uniform even in the same surface of the metal layer 102. Hence,there has been proposed the method in which the metal filling rate isequalized by forming recess portions in a ceramic sheet corresponding tothe area where the metal layer 102 should be formed (for example, referto Patent Document No. 3).

In the case of using the abovementioned multilayer piezoelectric elementas a piezoelectric actuator, it can be driven by connecting and securinglead wires (not shown) by soldering to the external electrodes 105,respectively, and then applying a predetermined potential to between theexternal electrodes 105. The multilayer piezoelectric element used forthis purpose is recently miniaturized and also required to ensure alarge displacement under large pressure. Hence, the abovementionedmultilayer piezoelectric element is required to be usable even undersevere conditions of higher electric field (voltage) application and along-term continuous driving.

In order to meet the abovementioned requirement, namely, the requirementof a long-term continuous driving under high voltage and high pressure,Patent Document No. 4 describes the element provided with a layer inwhich the thickness of the piezoelectric layer 101 is varied. That is,stress relaxation is performed utilizing the fact that the difference inthickness changes the displacement with respect to other layer.

In the simultaneous sintering type of multilayer piezoelectric element,attempts have been made to form a uniform metal layer so that a voltagecan be applied uniformly to every piezoelectric body. Particularly, inorder to equalize the electric conductivity of each metal layer, andequalize the surface area of the portion connected to the piezoelectricbody, attempts have been made to equalize the metal composition of themetal layer. Further, in order to equalize the surface area of theportion connected to the piezoelectric body, attempts have been made toequalize the thickness of the metal layers.

In the stacked type of multilayer piezoelectric element, it has beenproposed to control so that the contact resistance of the interfacebetween the electrode and the piezoelectric body is high at the centerin the stacking direction of the multilayer piezoelectric element, andis lowered toward the both ends, and so that no stress concentrates atthe center in the stacking direction of the multilayer piezoelectricelement (for example, refer to Patent Document No. 5).

However, unlike the normal multilayer electronic components such ascapacitors, the multilayer piezoelectric element itself continuouslycauses a dimensional change at the time of driving. Therefore, if all ofthe piezoelectric bodies are closely driven with the metal layer inbetween, the piezoelectric element will be integrally drivinglydeformed, so that the stress due to the deformation of the element isconcentrated at the outer peripheral portion of the center of theelement which expands at the time of compression and necks at the timeof spreading. When this multilayer piezoelectric element is subjected toa long-term continuous driving under high voltage and high pressure, forthe above reason, delamination might arise on the interface (thestacking interface) between the piezoelectric layer and the metal layer.Especially, stress concentrates on the interface between an active layercausing piezoelectric displacement and the inactive layer causing nopiezoelectric displacement, and this interface becomes the startingpoint of delamination.

In some cases, resonance phenomenon that the displacement behaviors ofthe respective piezoelectric layers match with each other is generatedwhich may cause beat sound, and harmonic signals of integral multiplesof driving frequency are generated which may cause noise composition.When the multilayer piezoelectric element causing continuous dimensionalchanges are driven for a long period of time, the element temperaturerises. When the energy of the temperature rise of the element exceedsheat release, there arises so-called hermorunaway phenomenon that theelement temperature is raised acceleratedly. This leads to the problemthat the piezoelectric body displacement is lowered as the temperatureis raised, and the piezoelectric body displacement is sharply lowered bythe fact that the piezoelectric layer has a higher temperature than theCurie point of the piezoelectric body. Hence, a metal layer having asmall specific resistance is needed for suppressing the elementtemperature rise.

Further, there is the feature that the piezoelectric body displacementchanges by environmental temperatures. Therefore, when the conventionalmultilayer piezoelectric element is used as an actuator for use in adriving element such as a fuel injector, the piezoelectric bodydisplacement might vary by the element temperature rise. That is, due tothe problem that the desired displacement varies gradually, thesuppression of displacement variations during the long-term continuousoperation and the improvement of durability have been demanded.

As a method of solving the above problem, the methods as described inthe above Patent Document No. 4 and Patent Document No. 5 have beenemployed, however, it cannot be said that the improvements aresufficient under severe conditions of a long-term continuous driving athigh voltage and high pressure. That is, stress may concentrate at theouter periphery of the center of the element, and the displacement mayvary by the occurrence of cracks and flaking.

-   Patent Document No. 1: Japanese Unexamined Patent Publication No.    61-133715-   Patent Document No. 2: Japanese Unexamined utility model Publication    No. 01-130568-   Patent Document No. 3: Japanese Unexamined Patent Publication No.    10-199750-   Patent Document No. 4: Japanese Unexamined Patent Publication No.    60-86880-   Patent Document No. 5: Japanese Unexamined Patent Publication No.    06-326370

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

It is an advantage of the present invention to provide a multilayerpiezoelectric element having a large displacement under high voltage andhigh pressure, and having excellent durability enabling the displacementto be suppressed even in a long-term continuous driving, and provide aninjector using the multilayer piezoelectric element.

Means for Solving the Problems

The present inventors have made tremendous research effort to solve theabovementioned problems and have completed the present invention basedon the following new fact. That is, when a plurality of metal layers ina multilayer piezoelectric element include a plurality of metal layershaving a different specific metal filling rate from oppositely disposedmetal layers adjacent to each other in the stacking direction, thestress exerted on the element can be dispersed, so that a largedisplacement can be obtained and resonance phenomena can also besuppressed. Hence, even in a long-term continuous driving under highvoltage and high pressure, the variations in displacement and thedelamination of the stacked portions can be suppressed, therebyobtaining a multilayer piezoelectric element having excellentdurability.

Specifically, in a multilayer piezoelectric element of the presentinvention in which a plurality of piezoelectric layers and a pluralityof metal layers are stacked alternately, a plurality of the metal layersinclude a plurality of low-filled metal layers having a lower fillingrate of metal composing the metal layers than oppositely disposed metallayers adjacent to each other in a stacking direction.

In a multilayer piezoelectric element of the present invention in whicha plurality of piezoelectric layers and a plurality of metal layers arestacked alternately, a plurality of the metal layers include a pluralityof high-filled metal layers having a higher filling rate of metalcomposing the metal layers than oppositely disposed metal layersadjacent to each other in the stacking direction.

In a multilayer piezoelectric element of the present invention in whicha plurality of piezoelectric layers and a plurality of metal layers arestacked alternately, an inactive layer composed of a piezoelectric bodyis formed at both sides in a stacking direction, and a metal layeradjacent to the inactive layer is a low-filled metal layer having alower metal filling rate than a metal filling rate in metal layersadjacent to each other in the stacking direction.

In a multilayer piezoelectric element of the present invention in whicha plurality of piezoelectric layers and a plurality of metal layers arestacked alternately, an inactive layer composed of a piezoelectric bodyis formed at both sides in a stacking direction, and a metal layeradjacent to the inactive layer is a high-filled metal layer having ahigher metal filling rate than a metal filling rate in metal layersadjacent to each other in the stacking direction.

The present inventors also have made tremendous research effort to solvethe abovementioned problems and have completed the present inventionbased on the following new fact. That is, when a plurality of metallayers in a multilayer piezoelectric element include a plurality ofmetal layers having a different thickness from oppositely disposed metallayers adjacent to each other in the stacking direction, the stressexerted on the element can be dispersed. This enables attainment of alarge displacement and also suppression of resonance phenomena. Hence,even in a long-term continuous driving under high voltage and highpressure, the variations in displacement and the delamination of thestacked portions can be suppressed, thereby obtaining a multilayerpiezoelectric element having excellent durability.

Specifically, in other multilayer piezoelectric element of the presentinvention in which a plurality of piezoelectric layers and a pluralityof metal layers are stacked alternately, a plurality of the metal layersinclude a plurality of thin metal layers having a smaller thickness thanoppositely disposed metal layers adjacent to each other in a stackingdirection.

In other multilayer piezoelectric element of the present invention inwhich a plurality of piezoelectric layers and a plurality of metallayers are stacked alternately, a plurality of the metal layers includea plurality of thick metal layers having a larger thickness thanoppositely disposed metal layers adjacent to each other in a stackingdirection.

In other multilayer piezoelectric element of the present invention inwhich a plurality of piezoelectric layers and a plurality of metallayers are stacked alternately, an inactive layer composed of apiezoelectric body is formed at both sides in a stacking direction, anda metal layer adjacent to the inactive layer is a thin metal layerhaving a smaller thickness than metal layers adjacent to each other inthe stacking direction.

In other multilayer piezoelectric element of the present invention inwhich a plurality of piezoelectric layers and a plurality of metallayers are stacked alternately, an inactive layer composed of apiezoelectric body is formed at both sides in a stacking direction, anda metal layer adjacent to the inactive layer is a thick metal layerhaving a larger thickness than metal layers adjacent to each other inthe stacking direction.

The present inventors also have made tremendous research effort to solvethe abovementioned problems and have completed the present inventionbased on the following new fact. That is, instead of being uniformcomposition of all of a plurality of metal layers composing mainly of analloy, as has been conventional, by containing a plurality of high-ratiometal layers having a higher ratio of a component constituting an alloythan oppositely disposed metal layers adjacent to each other, a largedisplacement can be obtained, and resonance phenomena can be suppressed.Hence, even in a long-term continuous driving under high voltage andhigh pressure, the variations in displacement and the delamination ofthe stacked portions can be suppressed, thereby obtaining a multilayerpiezoelectric element having excellent durability.

Specifically, in still other multilayer piezoelectric element of thepresent invention in which a plurality of piezoelectric layers and aplurality of metal layers composed mainly of an alloy are stackedalternately, a plurality of the metal layers include a plurality ofhigh-ratio metal layers having a higher ratio of a componentconstituting the alloy than oppositely disposed metal layers adjacent toeach other in a stacking direction.

In the present invention, a plurality of metal layers may consist onlyof an alloy, or alternatively, a part of the alloy may besingle-component metal.

In still other multilayer piezoelectric element of the present inventionin which a plurality of piezoelectric layers and a plurality of metallayers are stacked alternately, a plurality of the metal layers includea plurality of high-ratio metal layers having a higher ratio of at leasta component constituting the metal layer than oppositely disposed metallayers adjacent to each other in a stacking direction.

In still other multilayer piezoelectric element of the present inventionin which a plurality of piezoelectric layers and a plurality of metallayers are stacked alternately, a plurality of the metal layers includeat least two types of metal layers having different main components, aplurality of one type of which are disposed with a plurality ofdifferent metal layers in between.

An injector of the present invention includes a container having aninjection hole, and the abovementioned multilayer piezoelectric elementhoused in the container. The injector is configured so that a liquidfilled in the container is discharged from the injection hole by thedriving of the multilayer piezoelectric element.

Effects of the Invention

In accordance with the multilayer piezoelectric element of the presentinvention, the plurality of metal layers include a plurality ofpredetermined metal layers having a different metal filling rate fromthe oppositely disposed metal layers adjacent to each other in thestacking direction, so that the metal layers having differentdisplacement behaviors can be disposed in the element. That is, thepiezoelectric layer around the low-filled metal layer has a smalldisplacement, and the piezoelectric layer around the high-filled metallayer has a large displacement, so that locations having differentdisplacements can be arranged separately in the element. Thus, when themetal layers having different displacement behaviors are arrangedseparately in the element, the suppression of the element displacementdue to stress concentration can be relaxed, thereby increasing theentire displacement of the piezoelectric element. Additionally, becausethe stress concentration due to the piezoelectric element displacementcan be suppressed, the delamination of the stacked portions can besuppressed even in a long-term continuous driving under high voltage andhigh pressure. Further, the arrangement of a plurality of predeterminedmetal layers can suppress resonance phenomena to be generated when thedisplacements (dimensional changes) of the piezoelectric elements becomeidentical. This enables prevention of beat sound generation and alsoprevention of harmonic signal generation, thereby suppressing the noiseof control signals.

In accordance with other multilayer piezoelectric element of the presentinvention, the plurality of metal layers include a plurality ofpredetermined metal layers having a different thickness from theoppositely disposed metal layers adjacent to each other in the stackingdirection, so that the metal layers having different displacementbehaviors can be disposed in the element. That is, because the thinmetal layer can be easily deformed to absorb the local stress of thepiezoelectric body displacement, the piezoelectric layer around the thinmetal layer has a small displacement, so that locations having differentdisplacements can be arranged separately in the element. In addition,because the thick metal layer repels the local stress of thepiezoelectric body displacement without any deformation of the thickmetal layer, the piezoelectric layer around the thick metal layer has alarge displacement, so that locations having different displacements canbe arranged separately in the element. Thus, when the metal layershaving different displacements are arranged separately in the element,the suppression of the element deformation due to stress concentrationcan be relaxed, thereby increasing the entire displacement of thepiezoelectric element. Additionally, because the stress concentrationdue to the piezoelectric element displacement can be suppressed, thedelamination of the stacked portions can be suppressed even in along-term continuous driving under high voltage and high pressure.Further, the arrangement of a plurality of predetermined metal layerscan suppress resonance phenomena to be generated when the displacements(dimensional changes) of the piezoelectric elements become identical.This enables preventions of beat sound generation and harmonic signalgeneration, thereby suppressing the noise of control signals.

In accordance with still other multilayer piezoelectric element of thepresent invention, the plurality of metal layers include a plurality ofthe high-ratio metal layers having a higher ratio of a componentconstituting an alloy than the oppositely disposed metal layers adjacentto each other, so that the metal layers having different hardnesses canbe arranged partially, thereby dispersing the stress exerted on thepiezoelectric element. This enables relaxation of the suppression of theelement deformation due to stress concentration, thereby increasing theentire displacement of the piezoelectric element. Additionally, becausethe stress concentration due to the piezoelectric element deformationcan be suppressed, the delamination of the stacked portions can besuppressed even in a long-term continuous driving under high voltage andhigh pressure. Further, the arrangement of a plurality of the high-ratiometal layers can suppress resonance phenomena to be generated when thedisplacements of the piezoelectric elements (dimensional changes) becomeidentical. This enables preventions of beat sound generation andharmonic signal generation, thereby suppressing the noise of controlsignals.

Even if the multilayer piezoelectric element of the present invention iscontinuously driven, the desired displacement will not be effectivelychanged, enabling to provide the injector having excellent durabilityand high reliability.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1( a) is a perspective view showing a multilayer piezoelectricelement according to an embodiment of the present invention; FIG. 1( b)is a partial perspective view showing a stacked state of piezoelectriclayers and metal layers in FIG. 1( a);

FIG. 2 is a partially enlarged cross section showing a stacked structureof the piezoelectric element according to a first preferred embodiment;

FIG. 3 is a partially enlarged cross section showing a high-filled metallayer in the first preferred embodiment;

FIG. 4 is a partially enlarged cross section showing other stackedstructure in the first preferred embodiment;

FIG. 5 is a partially enlarged cross section showing other stackedstructure in the first preferred embodiment;

FIG. 6 is a schematic explanatory drawing for explaining voids of thepiezoelectric layer in the first preferred embodiment;

FIG. 7 is a partially enlarged cross section showing the stackedstructure of a multilayer piezoelectric element according to a secondpreferred embodiment;

FIG. 8 is a partially enlarged cross section showing the stackedstructure of a multilayer piezoelectric element according to a fifthpreferred embodiment;

FIG. 9 is a partially enlarged cross section showing a thick metal layerin the fifth preferred embodiment;

FIG. 10 is a partially enlarged cross section showing other stackedstructures in the fifth preferred embodiment;

FIG. 11 is a partially enlarged cross section showing other stackedstructure in the fifth preferred embodiment;

FIG. 12 is a schematic explanatory drawing for explaining voids of apiezoelectric layer in the fifth preferred embodiment;

FIG. 13 is a partially enlarged cross section showing the stackedstructure of a multilayer piezoelectric element according to a sixthpreferred embodiment;

FIG. 14 is a partially enlarged cross section showing the stackedstructure of a multilayer piezoelectric element according to a ninthpreferred embodiment;

FIG. 15 is a partially enlarged cross section showing the stackedstructure of a multilayer piezoelectric element according to a tenthpreferred embodiment;

FIG. 16 is a partially enlarged cross section showing the stackedstructure of a multilayer piezoelectric element according to an eleventhpreferred embodiment;

FIG. 17 is a graph showing the silver composition of the metal layer ofSample No. III-35 in Table 15 in an example;

FIG. 18 is a schematic cross section showing the stacked structure of ametal layer connected to a piezoelectric layer of a multilayerpiezoelectric element according to a twelfth preferred embodiment;

FIG. 19( a) is a perspective view showing a multilayer piezoelectricelement according to a thirteenth preferred embodiment; and 19(b) is apartial perspective view showing a stacked state of a piezoelectriclayer and a metal layer in FIG. 19( a);

FIG. 20 is a schematic cross section showing an injector according to anembodiment of the present invention;

FIG. 21( a) is a perspective view showing a conventional multilayerpiezoelectric element; and FIG. 21( b) is a partial perspective viewshowing the stacked state of piezoelectric layers and metal layers inFIG. 21( a); and

FIG. 22 is a partially enlarged cross section showing the stackedstructure in the conventional multilayer piezoelectric element; and

FIG. 23 is a partially enlarged cross section showing the stackedstructure in the conventional multilayer piezoelectric element.

PREFERRED EMBODIMENTS FOR CARRYING OUT THE INVENTION

<Multilayer Piezoelectric Element>

(First Preferred Embodiment)

A first preferred embodiment of the multilayer piezoelectric element ofthe present invention will be described in detail with reference to theaccompanying drawings. FIG. 1( a) is a perspective view showing amultilayer piezoelectric element according to the present embodiment,and FIG. 1( b) is a partial perspective view showing a stacked state ofpiezoelectric layers and metal layers in FIG. 1( a). FIG. 2 is apartially enlarged cross section showing a stacked structure of thepiezoelectric element according to the present embodiment. FIG. 3 is apartially enlarged cross section showing a high-filled metal layer inthe present embodiment. FIG. 4 is a partially enlarged cross sectionshowing other stacked structure in the present embodiment. FIG. 5 is apartially enlarged cross section showing other stacked structure in thepresent embodiment. FIG. 6 is a schematic explanatory drawing forexplaining voids of the piezoelectric layer in the present embodiment.

As shown in FIG. 1, the multilayer piezoelectric element of the presentembodiment has a stacked body 13 configured by alternately stacking aplurality of piezoelectric layers 11 and a plurality of metal layers 12.A pair of external electrodes 15 are disposed on the opposed sidesurfaces of the stacked body 13 (one of the external electrodes is notshown).

As shown in FIG. 1( b), the metal layers 12 are not formed on the entiremain surfaces of the piezoelectric layers 11, and thus being a so-calledpartial electrode structure. A plurality of the metal layers 12 of thepartial electrode structure are arranged to be exposed every other layerto the opposed side surfaces of the stacked body 13, respectively. Thisenables the metal layers 12 to be electrically connected by every otherlayer to the pair of external electrodes 15. The pair of externalelectrodes 15 may be formed on the adjacent side surfaces, respectively.

As shown in FIG. 1( a), inactive layers 14 formed by a piezoelectriclayer are stacked on both sides in the stacking direction of the stackedbody 13, respectively. When the multilayer piezoelectric element is usedas a piezoelectric actuator, lead wires may be connected and secured bysoldering to the pair of external electrodes 15, respectively, and thelead wires may be connected to an external voltage supply part,respectively. By applying a predetermined voltage from the externalvoltage supply part to between the metal layers 12 adjacent to eachother through the lead wires, each of the piezoelectric layers 11 isdisplaced by inverse piezoelectric effect. Specifically, because themetal layers 12 are formed by a metal material such as silver-palladiumalloy, etc. to be described later, the application of a predeterminedvoltage to each of the piezoelectric bodies 11 though the metal layers12 produces the action causing the piezoelectric bodies 11 to bedisplaced by inverse piezoelectric effect.

On the other hand, the inactive layers 14 causes no displacement whenthe voltage is applied, because one main surface thereof is providedwith the metal layer 12, and the other main surface is not provided withthe metal layer 12.

As shown in FIG. 2, a plurality of the metal layers 12 according to thepresent embodiment include a plurality of low-filled metal layers 12 bhaving a lower filling rate of metal composing the metal layers 12 thanoppositely disposed metal layers (metal layers 12 a) adjacent to eachother in the stacking direction. Consequently, the piezoelectric layersaround the low-filled metal layers 12 b have a small displacement, andthe piezoelectric layers around the metal layers 12 a having a highermetal filling rate than the low-filled metal layers 12 b have a largedisplacement. Thus, the metal layers having different displacements canbe arranged separately in the element, thereby increasing thedisplacement of the entire piezoelectric element. Additionally, thedelamination to be generated at the stacked portions can be suppressedeven in a long-term continuous driving under high voltage and highpressure. Further, resonance phenomena can be suppressed, enablingsuppression of beat sound generation. Furthermore, harmonic signalgeneration can also be suppressed, enabling suppression of the noise ofcontrol signals.

The drivingly deformed locations in the piezoelectric layers 11correspond to the portions sandwiched between the metal layers 12. It istherefore preferable to form the low-filled metal layers 12 b at theportions of the metal layers 12 which are overlapped with each otherwith the piezoelectric layer 11 in between. This surely suppressesresonance phenomena to be generated when the displacements (thedimensional changes) of the piezoelectric elements become identical.

Preferably, each of the low-filled metal layers 12 b may be disposedinterposing in between a plurality of different metal layers other thanthe low-filled metal layers 12 b. The different metal layers of thepresent embodiment are the metal layers 12 a as shown in FIG. 2, andhigh-filled metal layers 12 c as described later with reference to FIG.3. The low-filled metal layers 12 b have a lower metal filling rate thanthe different metal layers (the metal layers 12 a and the high-filledmetal layers 12 c). Consequently, the low-filled metal layers 12 b havegreater flexibility than the different metal layers, and therefore, whenstress is exerted thereon during driving, the layers 12 b can bedeformed for relaxing the stress (stress relaxing effect). That is, thelow-filled metal layers 12 b function as a stress relaxing layer.

Particularly, it is preferable in the present embodiment that theplurality of the low-filled metal layers 12 b be arranged regularly inthe stacking direction. This is because the regular arrangement of thestress relaxing layers is effective for dispersing the stress exerted onthe entire element. Preferably, the stacked body 13 is configured bystacking at least three layers of the piezoelectric layers 11, andincludes a part where the low-filled metal layers 12 b are repeatedlyarranged in a predetermined order.

The above expression that “the plurality of the low-filled metal layers12 b are regularly arranged in the stacking direction” includes the casewhere the layer number of the different metal layers (the metal layers12 a or the high-filled metal layers 12 c), which are present betweenthe low-filled metal layers 12 b, is identical for each area between thelow-filled metal layers 12 b, as well as the case where the layer numberof the different metal layers 12 existing between the low-filled metallayers 12 b approaches such a degree that the stress can be dispersedsubstantially uniformly in the stacking direction. Specifically, thelayer number of the different metal layers 12 existing between thelow-filled metal layers 12 b is within ±20% with respect to the averagevalue of the respective layer numbers, preferably within ±10% withrespect to the average value of the respective layer numbers, and morepreferably all be identical number.

The metal layers 12 a as the different metal layers are those having ahigher metal filling rate than the low-filled metal layer 12 b. Thesemetal layers 12 a are the main metal layers. The term “the main metallayers” means metal layers composed of a plurality of metal layershaving the identical metal filling rate in the metal layers 12, and alsohaving a larger layer number than the low-filled metal layers 12 b andthe high-filled metal layers 12 c. Preferably, the layer number of themetal layers 12 a as the main metal layer is not less than one third ofthe total metal layer number in the order of proximity to the averagemetal filling rate of all of the metal layers. The reason for this is asfollows. That is, the function required for the main metal layers 12 ais to stably function as electrodes for driving the multilayerpiezoelectric element. It is therefore required that the voltage appliedto the element is uniformly supplied so as to uniformly performpiezoelectric displacement. When the main metal layers 12 a constitutenot less than one third of the total metal layer number in the order ofproximity to the average metal filling rate of all of the metal layers,the voltage applied to the element can be supplied uniformly to each ofthe piezoelectric layers 11. Hence, without excessive non-uniformdriving deformation of the piezoelectric layers 11, the element can beapproximately uniformly drivingly deformed as a whole, resulting in theelement with durability. Additionally, the piezoelectric layers 11connected to the main metal layers 12 a are free from stressconcentration, permitting a large displacement. The piezoelectric layers11 connected to the low-filled metal layers 12 b become the stressrelaxing layers, thereby maintaining the driving displacement of theelement, and avoiding stress concentration at a point of the element.This provides a large displacement and excellent durability.

In order to equalize the phases of displacements and raise responsespeed, the main metal layers 12 a, in the order of proximity to theaverage metal filling rate of all of the metal layers, constitute 70%and above, preferably 80% and above, more preferably 90% and above, andstill more preferably 90 to 99% of the total metal layer number. Whenthe main metal layers 12 a constitute 90% and above of the entire metallayer number, the phases of displacements can be equalized thereby toachieve higher response speed. Above 99%, the phases are completelyequalized, and undesirably the element may cause beat sound. The totallayer number of the metal layers 12 may be arbitrarily determineddepending on the purpose, and no special limitation is imposed thereon.However, it is usually 2 to 10000 layers, and preferably 5 to 1000layers.

Preferably, the layer number of the main metal layers 12 a is thelargest in the plurality of the metal layers 12. This enables thevoltage applied to the element to be uniformly supplied to therespective piezoelectric layers 11, thereby eliminating the non-uniformdriving deformation of the piezoelectric layers 11. In addition, owingto the equalized phases of displacements, the elements havesubstantially a uniform driving deformation, resulting in the multilayerpiezoelectric element having high response speed as well as durability.

Preferably, the main metal layers 12 a are the metal layers except forthose having the highest metal filling rate, and ones having the lowestmetal filling rate in the metal layers 12. The reason for this is asfollows. That is, the stress exerted on the multilayer piezoelectricelement during driving tends to be applied to the piezoelectric layers11 in the vicinity of the metal layers 12 having the highest metalfilling rate. Hence, if the main metal layers 12 a are metal layersother than those having the highest metal filling rate, it is possibleto obtain the multilayer piezoelectric element with high durability, inwhich the metal layers 12 a and the piezoelectric layers 11 connected tothe metal layers 12 a are firmly adhered to each other. Further, owingto a small displacement of the piezoelectric layers 11 connected to themetal layers 12 having a low metal filling rate, if the main metallayers 12 a are metal layers other than those having the lowest metalfilling rate, there is no possibility that the displacement of themultilayer piezoelectric element becomes excessively small. That is, byusing, as the main metal layers 12 a, the metal layers other than thosehaving the highest metal filling rate and those having the lowest metalfilling rate, the multilayer piezoelectric element having a largedriving displacement and durability can be attained. Additionally, bychanging the metal filling rate of the metal layers 12, the magnitude ofdisplacements of the piezoelectric layers 11 can be controlled. Thiseliminates the necessity to change the thickness of the piezoelectriclayers 11, and provides excellent mass production. Preferably, the mainmetal layers 12 a (a plurality of the metal layers 12 a) havesubstantially the same metal filling rate. This leads to a largerdisplacement, higher response speed and improved durability.

Preferably, a plurality of the metal layers 12 include a plurality ofhigh-filled metal layers 12 c having a higher filling rate of the metalcomposing the metal layers 12 than oppositely disposed metal layersadjacent to each other in the stacking direction, as shown in FIG. 3.The reason for this is as follows. That is, since the high-filled metallayers 12 c having a high metal filling rate have less defected portionswhere no metal is filled in the metal layers, such as voids 12 c′, asshown in FIG. 6, the piezoelectric layers 11 connected to the metallayers 12 c become locations having a large displacement when a voltageis applied to the element. Therefore, when the element is driven, theselocations cause a large displacement, so that stress concentrates in thevicinity of the high-filled metal layers 12 c (stress concentrationeffect). By separately arranging such electrode layers in the element,the stress can be dispersed without any stress concentration at a pointin the element. This results in the multilayer piezoelectric elementhaving excellent durability and high reliability.

The high-filled metal layers 12 c have a high metal filling rate thanthe low-filled metal layers 12 b and the main metal layers 12 a. Thatis, the metal filling rates of the main metal layers 12 a, thelow-filled metal layers 12 b and the high-filled metal layers 12 c havethe following relationship: the high-filled metal layers 12 c>the mainmetal layers 12 a>the low-filled metal layers 12 b. Among all of themetal layers 12, the main metal layers 12 a correspond to the metallayers other than the metal layers having the highest metal filling rateand those having the lowest metal filling rate. This provides themultilayer piezoelectric element having a large driving displacement anddurability. This also ensures that the metal layers 12 having differentdisplacements are arranged in the element. As a result, thepiezoelectric layers 11 around the low-filled metal layers 12 b causes asmall displacement, and the piezoelectric layers 11 around thehigh-filled metal layers 12 c causes a large displacement. This permitsmore efficient achievement of the effect resulting from the arrangementof the metal layers having different displacements in the element.

Specifically, a filling rate ratio (Y1/X1) is in the range of 0.1 to0.9, preferably 0.3 to 0.9, and more preferably 0.5 to 0.8, where X1 isa metal filling rate in other metal layers except for the low-filledmetal layers 12 b and the high-filled metal layers 12 c (namely, themain metal layers 12 a), and Y1 is a metal filling rate in thelow-filled metal layers 12 b. This enables the stress relaxing effect ofthe low-filled metal layers 12 b to be obtained more surely, and alsoenables the element shape to be retained (preventing an excessive dropin the mechanical strength of the element). Especially, when the aboveratio (Y1/X1) is 0.3 to 0.9, the piezoelectric layers 11 adjacent to thelow-filled metal layers 12 b are also drivingly displaced, permittingthe multilayer piezoelectric element having a large displacement of theelement and having high durability. Further, when the above ratio(Y1/X1) is 0.5 to 0.8, it is possible to obtain the multilayerpiezoelectric element having a larger displacement of the element andhaving higher durability. The specific values of X1 and Y1 may bedetermined arbitrarily depending on the composition of the metal layers12, etc. Although no special limitation is imposed thereon, in general,X1 is 45 to 90%, preferably 55 to 85%, and more preferably 60 to 80%,and Y1 is 3 to 60%, preferably 20 to 60%, and more preferably 30 to 50%.It is preferable that X1 and Y1 be within the above range and satisfythe above ratio (Y1/X1).

On the other hand, when the above ratio (Y1/X1) is smaller than 0.1, thepiezoelectric layers 11 and the metal layers are hard to adhere to eachother, so that delamination might occur in the stacked body. Above 0.9,the stress relaxing effect of the low-filled metal layers 12 b might belowered, and there might appear a stress concentration point in theelement, and the durability of the element might be lowered.

Alternatively, a filling rate ratio (Z1/X1) is in the range of 1.05 to2, preferably 1.05 to 1.5, and more preferably 1.1 to 1.2, where X1 is ametal filling rate in different metal layers other than the low-filledmetal layers 12 b and the high-filled metal layers 12 c (namely, themain metal layers 12 a), and Z1 is a metal filling rate in thehigh-filled metal layers 12 c. This produces the stress relaxing effectof the high-filled metal layers 12 c, and also retains the elementshape. Especially, when the above ratio (Z1/X1) is 1.05 to 1.5, thepiezoelectric layers 11 adjacent to the high-filled metal layers 12 c,and the piezoelectric layers 11 adjacent to the main metal layers 12 aare also drivingly displaced almost similarly, thereby obtaining themultilayer piezoelectric element having high durability. Alternatively,when the above ratio (Z1/X1) is 1.1 to 1.2, the multilayer piezoelectricelement can have a larger displacement and high durability. Like X1 andY1 in the abovementioned ratio (Y1/X1), the specific values of X1 and Z1may be determined arbitrarily depending on the composition of the metallayers 12, etc. Although no special limitation is imposed thereon, ingeneral, X1 is 45 to 90%, preferably 55 to 85%, and more preferably 60to 80%, and Z1 is 60 to 100%, preferably 70 to 100%, and more preferably72 to 95%.

On the other hand, when the above ratio (Z1/X1) is larger than 2, stressmay concentrate on the high-filled metal layers 12 c, and the interfacebetween the high-filled metal layer 12 c and the piezoelectric layer 11may flake off, so that delamination might occur in the stacked body.Below 1.05, the stress concentration effect of the high-filled metallayers 12 c might be lowered, and there might appear a stressconcentration point in the element, so that the durability of theelement might be lowered.

The filling rate of metal composing the metal layers 12 is a measuredvalue of a surface obtained by cutting the multilayer piezoelectricelement in the stacking direction. Specifically, when the metal layers12 on the cut surface is observed with a scanning electron microscope(SEM) and a metal microscope, it can be seen that the metal layers 12are composed not only metal components but also elements other thanmetal, such as voids and ceramic composition, etc. Therefore, in thecross section of an arbitrary metal layer, the area of portionsconsisting only of metal is measured. A metal filling rate is obtainedby dividing the total area of the portions consisting only of the metalby the total area of this metal layer. By making similar measurements ofthe metal filling rates of the metal layers 12 a, the low-filled metallayers 12 b and the high-filled metal layers 12 c, the individual layerscan be discriminated.

It is preferable that the high-filled metal layer 12 c having a highermetal filling rate than the main metal layer 12 a, and the low-filledmetal layer 12 b having a lower metal filling rate than the main metallayer 12 a be oppositely disposed with the piezoelectric layer 11 inbetween, as shown in FIG. 4. Thus, the stress exerted on the elementduring driving can be concentrated on the plurality of the high-filledmetal layers 12 c having a high metal filling rate, respectively,thereby dispersing the stress exerted on the element. Further, thelow-filled metal layers 12 b having a low metal filling rate, serving asthe stress relaxing layer, are disposed adjacent to the high-filledmetal layers 12 c, enabling the stress exerted on the element to bedispersedly relaxed more efficiently.

Especially, the oppositely disposed metal layers adjacent to each otherin the stacking direction with respect to the low-filled metal layer 12b are preferably the high-filled metal layers 12 c, as shown in FIG. 5.Thus, the stress exerted on the element during driving can beconcentrated on the plurality of the high-filled metal layers 12 chaving a high metal filling rate, respectively, enabling the stressexerted on the element to be dispersedly relaxed. Further, thelow-filled metal layers 12 b having a low metal filling rate, serving asthe stress relaxing layer, are disposed adjacent to the both sides ofthe high-filled metal layer 12 c, thereby more surely dispersedlyrelaxing the stress exerted on the element. When the low-filled metallayers 12 b as the stress relaxing layer are sandwiched by thehigh-filled metal layers 12 c as the stress collecting layers, thestress can be confined within the low-filled metal layers 12 b, enablingthe stress of the entire element to be dispersedly relaxed. As a result,when the element is applied to a piezoelectric actuator, it is possibleto provide the piezoelectric actuator having excellent durability andhigh reliability. The layer number of the low-filled metal layers 12 bto be sandwiched is preferably one because a smaller layer numberproduces more stress confining effect.

Alternatively, it is preferable that the low-filled metal layers 12 b,the high-filled metal layers 12 c and the main metal layer 12 a bearranged in the order named and in the stacking direction of the stackedbody 13, with the piezoelectric layer in between, and the main metallayers 12 a be stacked in descending order of the metal filling rate.Thus, the stress in the element during driving can be concentrated onthe high-filled metal layers 12 c, enabling the stress exerted on theelement to be dispersed. Further, the low-filled metal layers 12 bserving as the stress relaxing layers are disposed adjacent to the metallayers which collect stress, enabling the stress exerted on the elementto be dispersedly relaxed. The main metal layers 12 a are also arrangedin descending order of the metal filling rate, enabling the stresscollected on the high-filed metal layers 12 c to be dispersed gradually.In addition, by increasing the metal filling rate, the displacement ofthe adjacent piezoelectric layers 11 can be increased, thereby achievingthe multilayer piezoelectric element having a large displacement,excellent durability and high reliability.

Preferably, the high-filled metal layers 12 c have a peak metal fillingrate, and there is a tilted region where the metal filling rate isgradually lowered from the high-filled metal layers 12 c, throughoutover two layers, preferably 2 to 5 layers and more in the stackingdirection. Thus, the stress in the element during driving concentrateson the high-filled metal layers 12 c. However, the presence of apredetermined tilted region enables the stress collected at thehigh-filled metal layers 12 c to be dispersed gradually.

Preferably, the metal layers 12 have predetermined voids 12 a′, 12 b′and 12 c′, as shown in FIG. 6. The reason for this is as follows. Thatis, if any insulating material other than the metal composition iscontained in the metal layers 12, when the element is driven, theportions to which no voltage can be applied may appear in thepiezoelectric layers 11. Therefore, piezoelectric displacement cannot beincreased, and the stress during driving concentrates at these metallayers 12, which might become the starting points of breakdown. If themetal layers 12 have the predetermined voids, when stress is exerted onthe metal portions, the presence of the areas of voids facilitates themetal deformation, enabling the stress to be effectively dispersedlyrelaxed. When the piezoelectric layers 11 connected to the metal layers12 cause piezoelectric displacement, the piezoelectric layers 11 can bepartially cramped by the presence of the void portions. Therefore, theforce constraining the piezoelectric layers 11 can be decreased than thecase of cramping by the entire surface, so that they are easy todisplace, thereby increasing their displacements. This provides themultilayer piezoelectric element having a larger displacement of theelement and having high durability.

Particularly, the main metal layers 12 a is provided with voids 12 a′,and the area ratio (the void ratio) of the voids 12 a′ to the entirecross-sectional area in the cross section of the metal layers 12 a is 5to 70%, preferably 7 to 70%, and more preferably 10 to 60%. This permitsa large displacement, thereby obtaining the multilayer piezoelectricelement having excellent displacement. Especially, when the void ratiois 7 to 70%, or 10 to 60%, the piezoelectric layers 11 can be moresmoothly deformed, and the displacement of the multilayer piezoelectricelement can be increased by the sufficient electric conductivity of themetal layers 12.

On the other hand, if the void ratio is smaller than 5%, thepiezoelectric layers 11 is constrained by the metal layers 12 when thepiezoelectric layers 11 are deformed by the applied voltage, therebysuppressing the deformation of the piezoelectric layers 11. This reducesthe amount of deformation of the multilayer piezoelectric element, andincreases the internal stress to be generated, so that durability mightbe affected. On the other hand, when the void ratio is larger than 70%,extremely narrow portions may occur at the electrode portions.Undesirably, the strength of the metal layers 12 themselves may belowered, and cracks are liable to occur in the metal layers 12, so thatdisconnection might occur.

The void ratio to the area of the metal layers 12 is a measured value ofa cross section obtained by cutting the multilayer piezoelectric elementby a plane parallel to the stacking direction, or a plane perpendicularto the stacking direction. Specifically, the measured value is obtainedby measuring the areas of voids existing in the metal layers 12 in thecut surface, and dividing the total of the void areas by the area of themetal layers 12, and then multiplying the result by 100.

More specific methods of measuring a void ratio are as follows. That is,the void ratio measuring method can be roughly classified into thefollowing two methods. A first method is the observation of the crosssection when the stacked body 13 is cut by a plane parallel to thestacking direction. A second method is the observation of the crosssection when the stacked body 13 is cut by a plane perpendicular to thestacking direction.

The void ratio measurement by the first method may be carried out asfollows. Firstly, by known polishing means, the stacked body 13 ispolished so that the cross section parallel to the stacking direction isexposed. For example, the stacked body 13 can be polished with diamondpaste by using, as a polisher, a bench polisher KEMET-V-300,manufactured by Kemet Japan Co., Ltd. The cross section exposed by thispolishing process is observed by, for example, a scanning electronmicroscope (SEM), an optical microscope, a metal microscope, etc,thereby obtaining a cross section image. The void ratio of the metallayers can be determined by performing image processing of the crosssection image. As a specific example, on the image of the metal layerstaken by the optical microscope, void portions are colored in black, andthe portions other than the voids are colored in white. Then, the ratioof the black portions, namely, (the area of the black portions)/(thearea of the black portions plus the area of the white portions), isfound, and the void ration can be calculated by expressing the result asa percentage. For example, when the cross section image is a colorimage, it may be converted to gray scales and divided into blackportions and white portions. At this time, if required to set thethreshold value of a boundary for converting into black portions andwhite portions, binarization may be carried out, setting the thresholdvalue of the boundary by image processing software and visualobservation.

The void ratio measurement by the second method may be carried out asfollows. Firstly, using a known polisher, the stacked body 13 ispolished until a cross section of the metal layer whose void ratiomeasurement is desired (a cross section perpendicular to the stackingdirection) is exposed. For example, the stacked body 13 can be polishedwith diamond paste by using, as a polisher, the bench polisherKEMET-V-300, manufactured by Kemet Japan Co., Ltd. The cross sectionexposed by this polishing process is observed by, for example, ascanning electron microscope (SEM), an optical microscope, a metalmicroscope, etc, thereby obtaining a cross section image. The void ratioof the metal layers can be determined by performing image processing ofthe cross section image. As a specific example, on the image of themetal layers taken by the optical microscope, void portions are coloredin black, and the portions other than the voids are colored in white.Then, the ratio of the black portions, namely, (the area of the blackportions)/(the area of the black portions plus the area of the whiteportions), is found, and the void ration can be calculated by expressingthe result as a percentage. For example, when the cross section image isa color image, it may be converted to gray scales and divided into blackportions and white portions. At this time, if required to set thethreshold value of a boundary for converting into black portions andwhite portions, binarization may be carried out, setting the thresholdvalue of the boundary by image processing software and visualobservation. When observing the cross sections of the metal layers, itis preferable to perform the polishing so as to reduce their thicknessesto substantially a half, and observe the cross section so exposed.However, if the metal layer has a small thickness and relatively largethickness variations, the entire cross section of the metal layer maynot be exposed by polishing process. In such a case, at the point thatthe polishing process is performed until part of the metal layer isexposed, the exposed portion is observed to obtain a cross sectionimage. Thereafter, the polishing is advanced, and the portions exceptfor the observed portions may be observed. This operation may berepeated a plurality of times. Thus, the observed images obtained by aplurality of the above operation may be combined together so as toattain the entire cross section of the metal layer.

The metal layers 12 having the abovementioned voids are composed mainlyof metal and voids. In the metal layers 12 so composed, both of themetal and the voids are deformable against stress, resulting in themultilayer piezoelectric element with higher durability.

Especially, when the low-filled metal layers 12 b are composed mainly ofmetal and voids, the multilayer piezoelectric element can have stillhigher durability. That is, as shown in FIG. 6, the low-filled metallayer 12 b is preferably composed of a plurality of metal parts spacedapart with voids 12 b′ in between. Thus, when the piezoelectric layers11 connected to the low-filled metal layer 12 b are connected to theportions not filled with metal, such as the voids 12 b′, in the metallayers, the piezoelectric body located at that portions causes nodisplacement even if a voltage is applied to the element, and causesdeformation when stress is exerted during driving, thereby relaxing thestress (stress relaxing effect). That is, the low-filled metal layer 12b composed of the metal parts functions as a stress relaxing layer.Accordingly, the piezoelectric layers 11 connected to these metal layershave a small driving displacement, thereby avoiding that the stressexerted on the element concentrates at a point. This results in themultilayer piezoelectric element having excellent durability and highreliability.

Specifically, the area ratio (the void ratio) of the voids 12 b′ to theentire cross-section area in the cross section of the low-filled metallayer 12 b is preferably 20 to 90%. This further increases displacement,achieving the multilayer piezoelectric element having excellentdisplacement.

Preferably, the metal layers 12 are composed mainly of metal selectedfrom elements in groups 8 to 11 of the periodic table. This is becausethe above metal composition having high heat resistance enablessimultaneous sintering of the piezoelectric layers 11 having a highsintering temperature and the metal layers 12. Hence, the externalelectrodes 15 can be manufactured at a sintering temperature lower thanthe sintering temperature of the piezoelectric layers 11, therebysuppressing severe mutual diffusion between the piezoelectric layers 11and the external electrodes 15.

It is further preferable to compose mainly of metal satisfying thefollowing relationship of: 0<M1≦15, 85≦M2<100, M1+M2=100, where M1 (% bymass) is a content of an element in the groups 8 to 10 of the periodictable in the metal layers 12, and M2 (% by mass) is a content of anelement in the group 11 of the periodic table. The reason for this is asfollows. When the M1 as the content of an element in the groups 8 to 10of the periodic table exceeds 15% by mass, specific resistance isincreased, and when the multilayer piezoelectric element is continuouslydriven, the metal layers 12 generate heat. The heat generation acts onthe piezoelectric layers 11 having temperature dependency thereby toreduce the displacement characteristic thereof, and in some cases, theelement displacement may become small. Further, when the externalelectrodes 15 are formed, the external electrodes 15 and the metallayers 12 are mutually diffused and connected to each other. However, ifthe M1 exceeds 15% by mass, this increases the hardness of locationswhere the metal layer composition is diffused into the externalelectrodes 15. Therefore, durability might be lowered in the multilayerpiezoelectric element causing dimensional changes during driving.

Particularly, for the purpose of suppressing ion migration of theelement of the group 11 in the metal layers 12 into the piezoelectriclayers 11, the M1 is preferably not less than 0.001% by mass nor morethan 15% by mass. For the purpose of improving the durability of themultilayer piezoelectric element, the M1 is preferably not less than0.1% by mass nor more than 10% by mass. When excellent thermalconduction and higher durability are required, the M1 is preferably notless than 0.5% by mass nor more than 9.5% by mass. When still higherdurability is required, the M1 is preferably not less than 2% by massnor more than 8% by mass.

On the other hand, when the M2 as the content of an element in the group11 is less than 85% by mass, the specific resistance of the metal layers12 is increased, and when the multilayer piezoelectric element iscontinuously driven, undesirably the metal layers 12 might generateheat. Particularly, for the purpose of suppressing the ion migration ofthe element of the group 11 in the metal layers 12 into thepiezoelectric layers 11, the M2 is preferably not less than 85% by massnor more than 99.999% by mass. For the purpose of improving thedurability of the multilayer piezoelectric element, the M2 is preferablynot less than 90% by mass nor more than 99.9% by mass. When higherdurability is required, the M2 is preferably not less than 90.5% by massnor more than 99.5% by mass. When still higher durability is required,the M2 is preferably not less than 92% by mass nor more than 98% bymass.

Particularly when the low-filled metal layers 12 b relax stress,relaxing the applied stress means to release the stress by convertingthe applied kinetic energy to thermal energy, and the stress releasingportion retains heat. As the temperature of the piezoelectric body israised, the force of piezoelectric displacement is reduced. Once thetemperature is raised to Curie point, polarization effect will bevanished even if cooled, and the force of piezoelectric displacement isgreatly impaired. Consequently, if the low-filled metal layers 12 b canplay the role of a heat sink, it will become possible to dissipate theheat from the stress relaxing portions to the outside of the element.

Here, the use of metal having the composition of the present embodimentincreases heat dissipation effect, enabling the stress relaxing effectto be retained with high durability for a long period of time.Particularly, the composition containing a high concentration of silverhaving high thermal conduction can produce the highest thermaldissipation effect. Further, even if oxidized, the thermal conductivitywill not be deteriorated, and electric conductivity will not also bedeteriorated, resulting in the stress relaxing layer with extremely highdurability.

The M1 as the element in the groups 8 to 10, and the M2 as an element inthe group 11, which express the % by mass of the metal composition inthe metal layers 12, can be specified by analysis method such as EPMA(Electron Probe Micro Analysis) method or the like, respectively.

In the metal composition in the metal layers 12, the element in thegroups 8 to 10 is preferably at least one selected from Ni, Pt, Pd, Rh,Ir, Ru and Os, and the element in the group 11 is preferably at leastone selected from Cu, Ag and Au. These illustrated metals axe metalcompositions having excellent mass production in the recent alloy powdersynthesizing techniques.

Among the above illustrated metal compositions in the metal layers 12,it is preferable that the metal of an element in the groups 8 to 10 isat least one selected from Pt and Pd, and the metal of an element in thegroup 11 is at least one selected from Ag and Au. This makes it possibleto form the metal layers 12 having excellent heat resistance and smallspecific resistance.

Especially, in the metal composition in the metal layers 12, the metalof an element in the groups 8 to 10 is preferably Ni. This makes itpossible to form the metal layers 12 having excellent heat resistance.The metal of an element in the group 11 is preferably Cu. This makes itpossible to form the metal layers 12 having low hardness and excellentheat conductivity.

Particularly, Cu has high thermal conductivity, as well as thecharacteristic feature that when stress is exerted from a certaindirection, crystal orientation is oriented in the certain direction inwhich the stress is applied, thereby producing strong stress relaxingeffect, by which no breakage may occur. Further, when the element ismanufactured by simultaneous sintering, a coating layer of CuO havingstrong corrosion resistance can be formed on the Cu surface, resultingin the element having high durability (with normal Cu metal, a Cu₂Ocoating is gradually formed on the surface and then bound with themoisture in the air, which forms patina, leading to corrosion).

Alternatively, the metal layers 12 are preferably alloys composed mainlyof the above metal. As an example of the alloys, a completelysolid-dissolved alloy, such as a silver-palladium alloy (70 to 99.999%by mass of silver and 0.001 to 30% by mass of palladium), is suitablebecause the sintering temperature can be controlled at an arbitrarycomposition ratio. It is also preferable to add oxide, nitride orcarbide together with the above metal composition into the metal layers12. This increases the strength of the metal layers 12, and improves thedurability of the multilayer piezoelectric element. Particularly, oxideis preferred because the mutual diffusion between the oxide and thepiezoelectric layers 11 can increase the adhesion strength between themetal layers 12 and the piezoelectric layers 11.

The oxide is preferably composed mainly of peroviskite-type oxideconsisting of PbZrO₃—PbTiO₃, because of its high adhesion strength withthe piezoelectric layers 11. The content of the added oxide and the likecan be calculated from the area ratio of the composition in the metallayers on a cross section SEM image of the multilayer piezoelectricelement.

Preferably, the abovementioned inorganic composition (namely, oxide,nitride or carbide to be added together with the metal composition) isnot more than 50 volume % to the metal. This can reduce the connectionstrength between the metal layers 12 and the piezoelectric layers 11than the strength of the piezoelectric layers 11. More preferably, it is30 volume %, thereby improving the durability of the multilayerpiezoelectric element.

The respective thicknesses of the metal layer 12 a, the low-filled metallayer 12 b and the high-filled metal layer 12 c, each constituting themetal layers 12, may be determined arbitrarily depending on thecomposition of the metal layers 12, etc, and no special limitation isimposed thereon. In general, the thickness of the metal layer 12 a is0.1 to 100 μm, preferably 0.5 to 10 μm, and more preferably about 1 to 5μm. The thickness of the low-filled metal layer 12 b is 0.05 to 100 μm,preferably about 0.1 to 10 μm, and more preferably about 0.5 to 5 μm.The thickness of the high-filled metal layer 12 c is 0.1 to 200 μm,preferably about 0.5 to 15 μm, and more preferably about 1 to 10 μm.

Preferably, the piezoelectric layers 11 are composed mainly ofperoviskite-type oxide. The reason for this is as follows. When thepiezoelectric layers 11 are formed by peroviskite-type piezoelectricceramics material represented by, for example, barium titanate (BaTiO₃)or the like, owing to a high piezoelectric distortion constant d₃₃indicating its piezoelectric characteristic, it is possible to increasedisplacement and sinter the piezoelectric layers 11 and the metal layers12 at the same time. The above-mentioned piezoelectric layers 11 arepreferably composed mainly of peroviskite-type oxide consisting ofPbZrO₃—PbTiO₃ having a relatively high piezoelectric distortion constantd₃₃.

Preferably, the metal layers 12 are exposed to the side surfaces of thestacked body 13. The reason for this is as follows. The locations wherethe metal layers 12 are not exposed to the element side surfaces cannotbe displaced during driving, and therefore the region causingdisplacements during driving will be confined in the inside of theelement. As a result, the stress at the time of displacement is liableto concentrate on the abovementioned interface. Undesirably, this maycause the problem of durability.

The stacked body 13 is preferably a polygon cylindrical body. The reasonfor this is as follows. That is, if the stacked body 13 has acylindrical shape, the central axis may dislocate unless it iscompletely rounded. It is therefore necessary to prepare high precisioncircles and stack them one upon another, making it difficult to use amass production type manufacturing method using simultaneous sintering.Alternatively, if the outer periphery is polished to a cylindrical shapeafter stacking substantially circular stacked bodies or after sintering,it becomes difficult to align the central axes of the metal layers 12with high precision. On the contrary, if the stacked body 13 is thepolygon cylindrical body, the metal layers 12 can be formed on thepiezoelectric layers 11 whose reference line is determined.Additionally, these can be stacked one upon another along the referenceline, enabling the central axis as the driving axis to be formed with amass production type manufacturing method. This achieves the elementhaving high durability.

As described above, the metal layer 12 whose end is exposed, and themetal layer 12 whose end is not exposed to the side surfaces of themultilayer piezoelectric element of the present embodiment are disposedalternately. It is preferable that a groove is formed in thepiezoelectric layer 11 between the metal layer 12 whose end is notexposed, and the external electrode 15, and that an insulator having alower Young's modulus than the piezoelectric layer 11 is formed in thegroove. Thus, the stress generated by the displacement during drivingcan be relaxed, thereby suppressing the heat generation of the metallayers 12 even if the multilayer piezoelectric element is continuouslydriven.

(Second Preferred Embodiment)

A second preferred embodiment related to a multilayer piezoelectricelement of the present invention will next be described with referenceto the drawing. FIG. 7 is a partially enlarged cross section showing thestacked structure of a multilayer piezoelectric element according to thepresent embodiment. In FIG. 7, similar or equivalent parts to theconfigurations of FIGS. 1 to 6 as described above have similar referencenumbers, and the description thereof is omitted. As shown in FIG. 7,like the above-mentioned first preferred embodiment, the multilayerpiezoelectric element of the second preferred embodiment is themultilayer piezoelectric element in which a plurality of piezoelectriclayers 11 and a plurality of metal layers 12 are stacked alternately.

A plurality of the metal layers 12 include a plurality of high-filledmetal layers 12 c having a higher filling rate of metal composing themetal layers 12 than oppositely disposed metal layers (metal layers 12a) adjacent to each other in the stacking direction. This configurationalso produces the same effect as the abovementioned first preferredembodiment, because the piezoelectric layers 11 around the high-filledmetal layers 12 c have a large displacement, and the piezoelectriclayers 11 around the main metal layers 12 a, having a smaller metalfilling rate than the high-filled metal layers 12 c, have a smalldisplacement. This results in the configuration where the metal layershaving different displacements are arranged in the element.

Like the first preferred embodiment as described above, a plurality ofthe high-filled metal layers 12 c of the present embodiment arepreferably disposed interposing in between a plurality of differentmetal layers other than the high-filled metal layers 12 c (namely, themain metal layers 12 a and the low-filled metal layers 12 b).Preferably, these high-filled metal layers 12 c are regularly arrangedin the stacking direction. Preferably, a plurality of the metal layers12 include a plurality of low-filled metal layers 12 b having a lowerfilling rate of metal composing the metal layers 12 than oppositelydisposed metal layers adjacent to each other in the stacking direction.

The configuration is otherwise similar to that described in the firstpreferred embodiment, and therefore the description thereof is omitted.

(Third Preferred Embodiment)

A third preferred embodiment related to a multilayer piezoelectricelement of the present invention will next be described. The multilayerpiezoelectric element of the present embodiment is one in which aplurality of piezoelectric layers 11 and a plurality of metal layers 12are stacked alternately. Inactive layers 14 composed of a piezoelectricbody are formed at both sides in the stacking direction, respectively.Metal layers 12 adjacent to the inactive layers 14 are low-filled metallayers (low-filled metal layers 12 b) having a lower filling rate of themetal in the metal layers 12 than the metal layers 12 adjacent to eachother in the stacking direction. This avoids that the stress exerted onthe element concentrates at a point. The reason for this seems to be asfollows.

That is, the inactive layers not sandwiched with electrodes will not bedrivingly deformed even if a voltage is applied. Therefore, a drivinglydeformed portion and a non-drivingly deformed portion are bounded by themetal layer 12 adjacent to the inactive layer 14. The stress thereforemay concentrate at this boundary portion. At this time, if all of themetal layers 12 have the same metal filling rate, the stress is liableto concentrate at a point in the above boundary portion. Hence,delamination might occur when the multilayer piezoelectric element iscontinuously driven under high voltage and high pressure for a longperiod of time.

When the metal filling rate of the metal layers 12 (the low-filled metallayers 12 b) adjacent to the inactive layers 14 is lower than the metalfilling rate of the metal layers 12 adjacent to each other in thestacking direction, the low-filled metal layers 12 b have greaterflexibility than other metal layers. Thus, when the element is drivenand the piezoelectric layers 11 are deformed, the low-filled metallayers 12 b themselves can be deformed thereby to relax the stress(stress relaxing effect). Further, because the inactive layers 14connected to the low-filled metal layers 12 b are formed by apiezoelectric material, the inactive layers 14 are deformable understress application, thereby relaxing the stress. That is, the low-filledmetal layers 12 b and the inactive layers 14 produce synergism of thestress relaxing effect. Additionally, since the low-filled metal layers12 b will deform themselves, the piezoelectric layer 11 sandwichedbetween the low-filled metal layer 12 b and the metal layer 12 adjacentthereto is subjected to both of the driving deformation due to voltageapplication, and the deformation due to stress application. Since thelow-filled metal layers 12 b will deform themselves for relaxing stress,the deformation due to stress application is dominant and hence thepiezoelectric layer 11 will deform for relaxing the stress.Consequently, the driving displacement becomes small, thereby avoidingthat the stress exerted on the element concentrates at a point.

Preferably, the metal layer adjacent to the low-filled metal layer 12 bin the stacking direction is the high-filled metal layer 12 c. Thisenables that the stress during the time the element is driven can beconcentrated on the high-filled metal layer 12 c, and the stress exertedon the element can be dispersed into the ends. Further, by arranging thelow-filled metal layer 12 b serving as a stress relaxing layer so as tobe adjacent to the metal layer that collects stress, the stress exertedon the element can be dispersedly relaxed into the ends. When thehigh-filled metal layer 12 c as a stress collecting layer, and theinactive layer 14 sandwich in between the low-filled metal layer 12 b asa stress relaxing layer, stress can be confined within the low-filledmetal layer 12 b, further improving the effect of dispersedly relaxingthe stress exerted on the entire element. As a result, when the elementis applied to a piezoelectric actuator, it is possible to provide thepiezoelectric actuator having excellent durability and high reliability.

Conventionally, especially when forming a multilayer piezoelectricelement having a stacking number of less than 50, for example, the metalcontent of the piezoelectric layer 11 in the vicinity of the inactivelayer 14 is increased as it approaches the inactive layer 14, in orderthat displacement can be suppressed to suppress the stress fromconcentrating at the boundary portion. For this, in order to form thepiezoelectric layers 11, piezoelectric sheets having several kinds ofmetal contents have to be prepared and stacked one upon another,resulting in a high cost product. On the other hand, in the presentinvention, the multilayer piezoelectric element having high durabilitycan be manufactured at a low lost, only by changing the metal layers 12adjacent to the inactive layers 14 into the low-filled metal layer 12 bhaving a lower metal filling rate than the metal layers 12 adjacent toeach other in the stacking direction. Further, a multilayerpiezoelectric element having high durability can be manufactured at alower cost by changing the metal layers 12 adjacent to the inactivelayers 14 at both sides, into the low-filled metal layer 12 b having alower metal filling rate than the metal layers 12 adjacent to each otherin the stacking direction.

On the other hand, in a multilayer piezoelectric element having a largestacking number, by including a plurality of low-filled metal layers 12b having a lower metal filling rate than oppositely disposed metallayers (the metal layers 12 a) adjacent to each other in the stackingdirection, the piezoelectric layers around the low-filled metal layer 12b can reduce their displacement because the low-filled metal layers areeasily deformed to absorb the local stress of piezoelectricdisplacement. This enables the metal layers having differentdisplacements to be separately arranged in the element. Thus, even ifthe element is continuously driven under high voltage and high pressurefor a long period of time, the suppression of the element deformationdue to stress concentration can be relaxed, suppressing delamination tobe generated at the stacking portions. In addition, because resonancephenomena can be suppressed, beat sound generation can be prevented.Furthermore, harmonic signal generation can be prevented, suppressingthe noise of control signal. The configuration is otherwise similar tothose described in the first and second preferred embodiments, andtherefore the description thereof is omitted.

(Fourth Preferred Embodiment)

A fourth preferred embodiment related to a multilayer piezoelectricelement of the present invention will be described below. The multilayerpiezoelectric element of the present embodiment is one in which aplurality of piezoelectric layers 11 and a plurality of metal layers 12are stacked alternately, and inactive layers 14 composed of apiezoelectric body are formed on both sides in the stacking direction,respectively, and the metal layers 12 adjacent to the inactive layers 14are metal layers (high-filled metal layers 12 c) having a higher metalfilling rate than the metal layers 12 adjacent to each other in thestacking direction. This realizes a multilayer piezoelectric elementhaving excellent durability and high reliability. The reason for thisseems to be as follows.

That is, since the inactive layers not sandwiched with electrodes willnot be drivingly deformed even if a voltage is applied, a drivinglydeformed portion and a non-drivingly deformed portion are bounded by themetal layers 12 adjacent to the inactive layers 14. The stress thereforeconcentrates at the boundary portion. At this time, if all of the metallayers 12 have the same metal filling rate, the stress is liable toconcentrate at a point in the above boundary portion. Therefore,delamination might occur when the multilayer piezoelectric element iscontinuously driven under high voltage and high pressure for a longperiod of time.

In a state where the metal filling rate in the metal layers 12 adjacentto the inactive layers 14 becomes higher than the metal filling rate inthe metal layers 12 adjacent to each other in the stacking direction,when the element is driven and the piezoelectric layers 11 are deformed,the high-filled metal layers 12 c repel any local stress ofpiezoelectric displacement without their deformation, because theyexhibit strong force constraining not only the inactive layers 14connected to the high-filled metal layers 12 c, but also thepiezoelectric layers 11 connected to the high-filled metal layers 12 c.Therefore, the piezoelectric layers 11 connected to the high-filledmetal layers 12 c cause a larger displacement. This permits an increasein the piezoelectric displacement of the element.

Further, when the element is driven, the high-filled metal layers 12 care not deformed themselves for the above reason, and therefore thestress exerted on the entire element concentrates in the vicinity of thehigh-filled metal layers 12 c (stress concentration effect). Thus, byarranging these high-filled metal layers 12 c at the ends of the drivingportion of the element, the stress can be dispersed into the ends of theelement, without any stress concentration into the driving portion ofthe element, permitting the multilayer piezoelectric element havingexcellent durability and high reliability.

Conventionally, especially when forming a multilayer piezoelectricelement having a stacking number of less than 50, for example, the metalcontent of the piezoelectric layer 11 in the vicinity of the inactivelayer 14 is increased as it approaches the inactive layer 14, in orderthat displacement can be suppressed to suppress the stress fromconcentrating at the boundary portion. For this, in order to form thepiezoelectric layers 11, piezoelectric sheets having several kinds ofmetal contents have to be prepared and stacked one upon another,resulting in a high cost product. On the other hand, in the presentinvention, the multilayer piezoelectric element having high drivingforce and high durability can be manufactured at a low lost, only bychanging the metal layers 12 into the metal layers (the high-filledmetal layers 12 c) having a higher metal filling rate than the metallayers 12 adjacent to each other in the stacking direction. Further, amultilayer piezoelectric element having high durability can bemanufactured at a lower cost by changing the metal layers 12 adjacent tothe inactive layers 14 at both sides into the low-filled metal layers(the low-filled metal layers 12 b) having a lower metal filling ratethan the metal layers 12 adjacent to each other in the stackingdirection. The configuration is otherwise similar to those described inthe first to third preferred embodiments, and therefore the descriptionthereof is omitted.

A description will next be made of a method of manufacturing themultilayer piezoelectric elements according to the first to fourthpreferred embodiments as described above.

Firstly, slurry is prepared by mixing the calcinated powder ofperoviskite-type oxide composed of PbZrO₃—PbTiO₃ or the like, bindercomposed of organic high polymer of acrylic, butyral or the like, andplasticizer such as DBP (dibutyl phthalate), DOP (dioctyl phthalate) orthe like. The slurry is then subjected to a known tape forming methodsuch as doctor blade method, calendar roll, thereby obtaining aplurality of ceramic green sheets serving as the piezoelectric layers11.

Subsequently, a conductive paste is prepared by containing an organicmatter such as acryl beads, which are bindingly fixed during drying, andvolatized during sintering, in metal powder composing the metal layers12, such as silver-palladium alloy, and by adding and mixing binder andplasticizer. The conductive paste is then printed in a thickness of 1 to40 μm on the upper surfaces of the respective green sheets by screenprinting or the like.

Here, the metal filling rate of the metal layer 12 is changeable bychanging the ratio of the organic matter and the metal powder. That is,the organic matter may vaporize during sintering, so that voids can beformed in the metal layers 12. Accordingly, a low content of the organicmatter increases the metal filling rate, and a high content of theorganic matter decreases the metal filling rate. Specific organiccontents of the metal layers 12 a to 12 c are as follows. The metallayer 12 a is 0.1 to 10 parts by mass, and preferably 1 to 5 parts bymass with respect to 100 parts by mass of metal powder. The low-filledmetal layer 12 b is 0.1 to 50 parts by mass, and preferably 2 to 10parts by mass with respect to 100 parts by mass of metal powder. Thehigh-filled metal layer 12 c is 0 to 5 parts by mass, and preferably 0to 2 parts by mass with respect to 100 parts by mass of metal powder.

Although no special limitation is imposed on the organic matter, as longas it exhibits good thermally dissolving behavior during sintering, theabovementioned acryl beads and resin beads of acryl or α-methyl styreneresin are preferred. The acryl beads and the resin beads may have ahollow structure. Preferably, the acryl beads and the resin beads have amean particle size of approximately 0.01 to 3 μm.

Alternatively, acryl beads paste may be prepared by adding while mixingbinder and plasticizer to an organic matter such as acryl beads. Aconductive paste is prepared by adding while mixing binder andplasticizer to metal powder composing the metal layers 12, such assilver-palladium. The acryl beads paste and the conductive paste arestackingly printed on the upper surfaces of the respective green sheetsby screen printing or the like. This enables printing having moreexcellent mass production.

Subsequently, a plurality of the green sheets with the conductive pasteprinted thereon are stacked one upon another. The stacked body with aheavy stone mounted thereon is debindered at a predeterminedtemperature. Thereafter, this is sintered without mounting any heavystone thereon so that voids can be formed in the metal layers, therebyobtaining the stacked body 13. The sintering temperature is 900 to 1200°C., and preferably 900 to 1000° C. The reason for this is as follows.When the sintering temperature is below 900° C., the sinteringtemperature is low and the sintering is insufficient, making itdifficult to manufacture a dense piezoelectric body. When the sinteringtemperature is above 1200° C., the connecting strength between the metallayer and the piezoelectric body becomes large.

At this time, by adding metal powder composing the metal layer 12, suchas silver-palladium, into the green sheets composing the inactive layers14, or alternatively by printing, on the green sheets, slurry consistingof metal powder composing the meta layers 12, such as silver-palladium,and an inorganic compound and binder, the shrinkage behavior andshrinkage during sintering of the inactive layers 14 and other portionscan approach each other, thereby forming a dense stacked body 13.

The stacked body 13 should not be limited to that manufactured by theabove manufacturing method, and it may be formed by any manufacturingmethod capable of forming the stacked body 13, in which a plurality ofthe piezoelectric layers 11 and a plurality of the metal layers 12 arestacked alternately.

Thereafter, the metal layer 12 whose end is exposed to the side surfaceof the multilayer piezoelectric element, and the metal layer 12 whoseend is not exposed thereto are alternately formed. Then, a groove isformed in a piezoelectric portion between the metal layer 12 whose endis not exposed, and the external electrodes 15. An insulator of resin orrubber, having a lower Young's modulus than the piezoelectric layers 11,is formed in the groove. Here, the groove is formed of the side surfaceof the stacked body 13 by using an internal dicing device or the like.

Next, a conductive silver glass paste is prepared by adding binder toglass powder. This is formed in a sheet, and the raw density of thesheet after drying (causing the solvent to splash) is controlled to 6 to9 g/cm³. This sheet is then transferred onto an external electrodeforming surface of a columnar stacked body 13, followed by baking at atemperature that is higher than the softening point of glass, and belowthe melting point (965° C.) of silver, and below fourth five of thesintering temperature of the stacked body 13. This enables splash andelimination of the binder composition in the sheets manufactured byusing the silver glass conductive paste, thereby forming the externalelectrodes 15 composed of porous conductive material having athree-dimensional mesh structure.

Here, the paste composing the external electrodes 15 may be stacked on amultilayer sheet and then baked, or stacked alternately per layer andthen baked. It is however excellent in mass production to perform bakingat a time after stacking on the multilayer sheet. In the case ofchanging the glass composition layer by layer, the amount of the glasscomposition may be changed sheet by sheet. If desired to form anextremely thin glass rich layer on the surface most adjacent to thepiezoelectric layer 11, a glass rich paste may be printed on the stackedbody 13 by screen printing or the like, and a multilayer sheet may bestacked thereon. Instead of the printing, a sheet of below 5 μm may beused.

The baking temperature of the above silver glass conductive paste isdesirably 500 to 800° C., from the point that a neck portion (theportion where crystal grains are collected) is effectively formed, thesilver in the silver glass conductive paste and the metal layer 12 arediffusedly connected to each other, the voids in the external electrodes15 are effectively retained, and the external electrodes 15 and the sidesurfaces of the columnar stacked body 13 are partially connected to eachother. The softening point of the glass composition in the silver glassconductive paste is desirably 500 to 800° C.

On the other hand, when the baking temperature is higher than 800° C.,the silver powder of the silver glass conductive paste is too advanced,and the porous conductive material having a three-dimensional meshstructure cannot be formed, so that the external electrodes 15 are toodense. As a result, the Young's modulus of the external electrodes 15 istoo high, and the stress during driving cannot be absorbed sufficiently,so that the external electrodes 15 might be disconnected. Preferably,the baking is performed at temperatures within 1.2 times of the glasssoftening point. When the baking temperature is lower than 500° C., asufficient diffused connection between the ends of the metal layer 12and the external electrodes 15 cannot be made, and no neck portion canbe formed, which might cause spark between the metal layer 12 and theexternal electrodes 15 during driving.

Next, silicone rubber is filled into the groove of the stacked body 13by immersing the stacked body 13 provided with the external electrodes15 in a silicone rubber solution, and then deaerating the siliconerubber solution in the vacuum. The stacked body 13 is then lifted fromthe silicone rubber solution, and the silicone rubber is coated on theside surfaces of the stacked body 13. The silicone rubber, which isfilled into the groove and also coated on the side surfaces of thestacked body 13, is then cured, thereby obtaining the multilayerpiezoelectric element.

When this multilayer piezoelectric element is used in a piezoelectricactuator, the polarization processing of the stacked body 13 isperformed by connecting lead wires to the external electrodes 15,respectively, and by applying through the lead wires a dc voltage of 0.1to 3 kV/mm to a pair of the external electrodes 15, respectively. Thisachieves a piezoelectric actuator using the multilayer piezoelectricelement of the present invention.

By connecting the lead wires of this piezoelectric actuator to anexternal voltage supply part, respectively, and applying a voltage tothe metal layers 12 through the lead wires and the external electrodes15, the respective piezoelectric layers 11 are greatly displaced by thereverse piezoelectric effect, thereby functioning as, for example, anautomobile fuel injection valve for performing fuel injection supply toan engine. Additionally, as this piezoelectric actuator is provided withthe multilayer piezoelectric element of the present invention, it has alarge displacement under high voltage and high pressure, and variationsof the displacement can be suppressed even under a long-term continuousdriving. In the present invention, the term “high voltage and highpressure” means to apply an alternating voltage of 0 to +300V to thepiezoelectric actuator (the multilayer piezoelectric element) at roomtemperature and at a frequency of 1 to 300 Hz.

Alternatively, a conductive auxiliary member composed of conductiveadhesive, in which a metal mesh or a mesh-shaped metal plate is buried,may be formed on the outer surfaces of the external electrodes 15. Inthis case, when a large current is inputted into the actuator so as tobe driven at high speed by disposing the conductive auxiliary member onthe outer surfaces of the external electrodes 15, the large current canbe admitted in the conductive auxiliary member, reducing the currentpassing through the external electrodes 15. For this reason, it ispossible to prevent that the external electrodes 15 will locallygenerate heat and cause disconnection, enabling significant improvementof durability. Additionally, as the metal mesh or the mesh-shaped metalplate is buried in the conductive adhesive, it is possible to preventthe occurrence of cracks in the conductive adhesive. The metal meshmeans one in which a metal line is knitted. The mesh-shaped metal platemeans one in which holes are formed in a metal plate so as to have theshape of a mesh.

The conductive adhesive constituting the conductive auxiliary member ispreferably composed of polyimide resin where silver powder is dispersed.That is, by dispersing silver powder having a low specific resistanceinto polyimide resin having high thermal resistance, it is possible toform a conductive auxiliary member having a low resistance value andmaintaining high adhesive strength if used at high temperature.

The conductive particles are preferably aspherical particles such asflake shape and needle shape. The reason for this is as follows. Thatis, by changing the shape of the conductive particles into particles ofaspherical shape such as flake shape and needle shape, the entanglementbetween the conductive particles can be made strong, further increasingthe shear strength of the conductive adhesive.

The multilayer piezoelectric elements of the present invention are notbe limited to these but are susceptible of various changes withoutdeparting from the gist of the present invention. For example, althoughthe foregoing preferred embodiments have described the cases where theexternal electrodes 15 are formed on the opposed side surfaces of thestacked body 13, in the present invention, the external electrodes 15may be formed, for example, on the adjacent side surfaces.

(Fifth Preferred Embodiment)

A fifth preferred embodiment related to a multilayer piezoelectricelement of the present invention will next be described in detail withreference to the accompanying drawings. FIG. 8 is a partially enlargedcross section showing the stacked structure of a multilayerpiezoelectric element according to the present embodiment. FIG. 9 is apartially enlarged cross section showing a thick metal layer in thepresent embodiment. FIG. 10 is a partially enlarged cross sectionshowing other stacked structures in the present embodiment. FIG. 11 is apartially enlarged cross section showing other stacked structure in thepresent embodiment. FIG. 12 is a schematic explanatory drawing forexplaining voids of a piezoelectric layer in the present embodiment. InFIGS. 8 to 12, similar or equivalent parts to the configurations ofFIGS. 1 to 7 as described above have similar reference numbers, and thedescription thereof is omitted.

A plurality of metal layers 12 according to the present embodimentinclude a plurality of thin metal layers 12 e having a smaller thicknessthan oppositely disposed metal layers (metal layers 12 d) adjacent toeach other in the stacking direction, as shown in FIG. 8. The thin metallayers 12 e can be easily deformed to absorb the local stress of thepiezoelectric body displacement. Hence, the piezoelectric layers 11around the thin metal layer 12 e have a small displacement. In addition,the piezoelectric layers 11 around thick metal layers 12 f describedlater (refer to FIG. 9) having a larger thickness than the thin metallayers 12 e have a large displacement because the hard-to-deform thickmetal layer repels the local stress of the piezoelectric bodydisplacement. Therefore, metal layers having different displacements canbe arranged separately in the element. This increases the displacementof the entire piezoelectric element, and also relaxes the suppression ofthe element deformation due to stress concentration even in along-period continuous driving under high voltage and high pressure,thereby suppressing delamination to be generated at the stackingportions. Further, resonance phenomena can also be suppressed, therebypreventing beat sound generation. Further, harmonic signal generationcan be prevented, thereby suppressing the noise of control signals.Furthermore, by changing the thicknesses of the metal layers 12 with amanufacturing method such as printing, the element having stressrelaxing effect can be manufactured without changing the thicknesses ofthe piezoelectric layers 11. This realizes the structure havingexcellent mass production.

Drivingly deformed portions of a plurality of the piezoelectric layers11 are the portions sandwiched with the metal layers 12. It is thereforepreferable to form the thin metal layers 12 e at the portions of aplurality of the metal layers 12 which are overlapped with each otherwith the piezoelectric layer 11 in between. This surely suppressesresonance phenomena to be generated when the displacements (dimensionalchanges) of the piezoelectric elements become identical.

Preferably, a plurality of the thin metal layers 12 e are respectivelydisposed interposing in between a plurality of different metal layershaving a larger thickness than the thin metal layers 12 e. The differentmetal layers of the present embodiment are the metal layers 12 d asshown in FIG. 8, and the thick metal layers 12 f described later asshown in FIG. 9. Here, the thin metal layers 12 e have a smallerthickness than the different metal layers (the metal layers 12 d and thethick metal layers 12 f). Hence, the thin metal layers 12 e have greaterflexibility than the different metal layers, and therefore, when theelement is driven and the piezoelectric layers 11 are deformed, the thinmetal layers 12 b can be deformed themselves for relaxing the stress(stress relaxing effect). That is, the thin metal layers 12 e functionas a stress relaxing layer. In the piezoelectric layers 11 connected tothe thin metal layers 12 e, the driving deformation due to voltageapplication, and the deformation due to stress application coexist. Thethin metal layers 12 e will deform themselves for relaxing stress.Therefore, the deformation due to stress application is dominant,thereby permitting deformation for relaxing the stress. Consequently,the driving displacement becomes small, thereby avoiding that the stressexerted on the element concentrates at a point. This achieves themultilayer piezoelectric element having excellent durability and highreliability.

Particularly, if a plurality of the thick metal layers 12 f areinterposed, the thick metal layers 12 f have strong force constrainingthe piezoelectric layers 11 connected to the thick metal layers 12 f,and repel the local stress of the piezoelectric displacement without anylarge deformation of the thick metal layers 12 f. Consequently, thepiezoelectric layers 11 connected to the thick metal layers 12 f cause astronger displacement. This permits an increase in the piezoelectricdisplacement of the element. In addition, when the element is driven,the thick metal layers 12 f are hard to deform themselves for the abovereason, so that the stress exerted on the entire element concentrates inthe vicinity of the thick metal layers 12 f (stress concentrationeffect). Thus, the stress concentrating portions are disposed locally inthe element, and the stress concentrating portions are surrounded by thethin metal layers 12 e having the stress relaxing effect. This enablesthe element to have extremely large stress relaxing effect as a whole.

Particularly, in the present embodiment, it is preferable that theplurality of the thin metal layers 12 e be regularly arranged in thestacking direction. This is because the regular arrangement of thestress relaxing layers is effective for dispersing the stress exerted onthe entire element. It is also preferable that the stacked body 13 beconfigured by stacking at least three layers of the piezoelectric layers11, and the thin metal layers 12 e be repetitively arranged in apredetermined order.

The above expression that the plurality of the thin metal layers 12 eare regularly arranged in the stacking direction includes the case wherethe layer number of the different metal layers (the metal layers 12 dand the thick metal layers 12 f), which are present between the thinmetal layers 12 e, is identical for each area between the thin metallayers 12 e, as well as the case where the layer number of the differentmetal layers 12 existing between the thin metal layers 12 e approachessuch a degree that the stress can be dispersed substantially uniformlyin the stacking direction. Specifically, the layer number of thedifferent metal layers 12 existing between the thin metal layers 12 e iswithin ±20% with respect to the average value of the respective layernumbers, preferably within ±10% with respect to the average value of therespective layer numbers, and more preferably all be identical number.

The metal layers 12 d as the different metal layers are the metal layerwhose thickness is larger than the thin metal layers 12 e. The metallayers 12 d are main metal layers. The term “the main meta layers” meansmetal layers which are composed of a plurality of metal layers havingthe identical thickness in the metal layers 12, and which have a largerlayer number than the thin metal layers 12 e and the thick metal layers12 f. Preferably, the layer number of the metal layers 12 d as the mainmetal layers is not less than one third of the total metal layer numberin the order of proximity to the average thickness of all of the metallayers. The reason for this is as follows. The function required for themain metal layers 12 d is to stably function as electrodes for drivingthe multilayer piezoelectric element. It is therefore required that thevoltage applied to the element is uniformly supplied so as to uniformlycause piezoelectric displacements. When the main metal layers 12 dconstitute not less than one third of the total metal layer number inthe order of proximity to the average thickness of all of the metallayers, the voltage applied to the element can be supplied uniformly toeach of the piezoelectric layers 11. Therefore, without excessivenon-uniform driving deformation of the piezoelectric layers 11, theelement can be approximately uniformly drivingly deformed as a whole,resulting in the element with durability. Additionally, thepiezoelectric layers 11 connected to the thin metal layers 12 e are freefrom stress concentration, permitting a large displacement. Thepiezoelectric layers 11 connected to the thin metal layers 12 e becomestress relaxing layers, thereby maintaining the driving displacement ofthe element, and avoiding stress concentration at a point of theelement. This provides a large displacement and excellent durability.

In order to equalize the phases of displacements and raise responsespeed, the main metal layers 12 d, in the order of proximity to theaverage thickness of all of the metal layers, constitute 70% and above,preferably 80% and above, more preferably 90% and above, and still morepreferably 90 to 99% of the total metal layer number. When the mainmetal layers 12 d constitute 90% and above of the entire metal layernumber, the phases of displacements can be equalized thereby to achievehigher response speed. Above 99%, the phases are completely equalized,and undesirably the element may cause beat sound.

Preferably, the layer number of the main metal layers 12 d is thelargest in the plurality of the metal layers 12. This enables thevoltage applied to the element to be uniformly supplied to therespective piezoelectric layers 11, thereby eliminating the non-uniformdriving deformation of the piezoelectric layers 11. In addition, owingto the equalized phases of displacements, the element can havesubstantially uniform driving deformation, thus achieving the multilayerpiezoelectric element having high response speed along with durability.

Preferably, the main metal layers 12 d are metal layers except for thosehaving the largest thickness and those having the smallest thickness inthe metal layers 12. The reason for this is as follows. That is, thestress exerted on the multilayer piezoelectric element during drivingtends to be applied to the piezoelectric layers 11 in the vicinity ofthe metal layers 12 having the largest thickness. Hence, if the mainmetal layers 12 d are metal layers other than those having the largestthickness, it is possible to obtain the multilayer piezoelectric elementwith high durability in which the metal layers 12 d and thepiezoelectric layers 11 connected to the metal layers 12 d are firmlyadhered to each other. Further, owing to a small displacement of thepiezoelectric layers 11 connected to the metal layers 12 having a smallthickness, if the main metal layers 12 d are metal layers other thanthose layers having the smallest thickness, there is no possibility thatthe displacement of the multilayer piezoelectric element becomesexcessively small. That is, by using, as the main metal layers 12 d, themetal layers other than those having the largest thickness and thosehaving the smallest thickness, it is capable of obtaining the multilayerpiezoelectric element having a large driving displacement anddurability. Additionally, by changing the thicknesses of the metallayers 12, the magnitude of displacements of the piezoelectric layers 11can be controlled, thereby eliminating the necessity to change thethicknesses of the piezoelectric layers 11. This permits excellent massproduction.

Preferably, a plurality of the metal layers 12 includes a plurality ofthick metal layers 12 f having a larger thickness than oppositelydisposed metal layers (metal layers 12 d) adjacent to each other in thestacking direction, as shown in FIG. 9. Thus, when the element is drivenand the piezoelectric layers 11 are deformed, the thick metal layers 12f have strong force constraining the piezoelectric layers 11 connectedto the thick metal layers 12 f, and repel the local stress of thepiezoelectric displacement without any large deformation of the thickmetal layers 12 f. Consequently, the piezoelectric layers 11 connectedto the thick metal layers 12 f cause a stronger displacement. Thispermits an increase in the piezoelectric displacement of the element. Inaddition, when the element is driven, the thick metal layers 12 f arehard to deform themselves for the above reason, so that the stressexerted on the entire element concentrates in the vicinity of the thickmetal layers 12 f (stress concentration effect). Accordingly, thearrangement of the thick metal layers 12 f in the element can avoidstress concentrating at a point in the element, and disperse the stress,achieving the multilayer piezoelectric element having excellentdurability and high reliability.

Drivingly deformed portions of a plurality of the piezoelectric layers11 are the portions sandwiched with the metal layers 12. It is thereforepreferable to form the thick metal layers 12 f at the portions of aplurality of the metal layers 12 which are overlapped with each otherwith the piezoelectric layers 11 in between. This further increases theeffect that the stress exerted on the entire element concentrates in thevicinity of the thick metal layers 12 f.

Particularly, if a plurality of the thin metal layers 12 e areinterposed, the thin metal layers 12 e have greater flexibility than thedifferent metal layers, and therefore, when the element is driven andthe piezoelectric layers are deformed, the thin metal layers 12 e deformthemselves for relaxing the stress (stress relaxing effect). That is,the thin metal layers 12 e function as a stress-relaxing layer. Althoughthe piezoelectric layers 11 connected to the thin metal layers 12 e aresubjected to both of the driving deformation due to voltage application,and the deformation due to stress application, the thin metal layers 12e will deform themselves for relaxing stress, so that the deformationdue to stress application is dominant. Hence, under the deformation forrelaxing the stress, the driving displacement becomes small, therebyavoiding that the stress exerted on the element concentrates at a point.Thus, the portions at which the stress concentrates are disposed locallyin the element, and the stress concentrating portions are surrounded bythe thin metal layers 12 e having the stress relaxing effect. Thisenables the element to have extremely large stress relaxing effect and alarge driving torque as a whole.

Particularly, in the present embodiment, it is preferable that theplurality of the thick metal layers 12 f be regularly arranged in thestacking direction. This is because the regular arrangement of thestress relaxing layers is effective for dispersing the stress exerted onthe entire element. It is also preferable that the stacked body 13 beconfigured by stacking at least three layers of the piezoelectric layers11, and there be a part where the thick metal layers 12 f arerepetitively arranged in a predetermined order.

The above expression that the plurality of the thick metal layers 12 fare regularly arranged in the stacking direction includes the case wherethe layer number of the different metal layers (the metal layers 12 dand the thin metal layers 12 e), which are present between the thickmetal layers 12 f, is identical for each area between the thick metallayers 12 f, as well as the case where the layer number of the differentmetal layers 12 existing between the thick metal layers 12 f approachessuch a degree that the stress can be dispersed substantially uniformlyin the stacking direction. Specifically, the layer number of thedifferent metal layers 12 existing between the thick metal layers 12 fis within ±20% with respect to the average value of the respective layernumbers, preferably within ±10% with respect to the average value of therespective layer numbers, and more preferably all be identical number.

The thick metal layers 12 f are metal layers having a larger thicknessthan the thin metal layers 12 e and the main metal layers 12 d. That is,the respective thicknesses of the main metal layers 12 d, the thin metallayers 12 e and the thick metal layers 12 f have the followingrelationship: the thick metal layers 12 f>the main metal layers 12 d>thethin metal layers 12 e. Among all of the metal layers 12, the main metallayers 12 d are the metal layers other than those having the largestthickness and those having the smallest thickness. This provides themultilayer piezoelectric element having a large driving displacement anddurability. This also ensures that the metal layers 12 having differentdisplacements are arranged in the element. As a result, thepiezoelectric layers 11 around the thin metal layers 12 e have a smalldisplacement, and the piezoelectric layers 11 around the thick metallayers 12 f have a large displacement. This permits more efficientachievement of the effect resulting from the arrangement of the metallayers having different displacements in the element.

Specifically, a thickness ratio (Y2/X2) is in the range of 0.1 to 0.9,preferably 0.3 to 0.9, and more preferably 0.5 to 0.8, where X2 is athickness of other metal layer except for the thick metal layer 12 e andthe thick metal layer 12 f (namely, the main metal layer 12 d), and Y2is a thickness of the thin metal layer 12 e. This enables the stressrelaxing effect of the thin metal layers 12 e to be obtained moresurely, and also enables the element shape to be retained (preventing anexcessive drop in the mechanical strength of the element). Especially,when the above ratio (Y2/X2) is 0.3 to 0.9, the piezoelectric layers 11adjacent to the thin metal layers 12 e are also drivingly displaced,thereby obtaining the multilayer piezoelectric element having a largedisplacement of the element and having high durability. Further, whenthe above ratio (Y2/X2) is 0.5 to 0.8, it is possible to obtain themultilayer piezoelectric element having a larger displacement of theelement and having higher durability. The specific values of X2 and Y2may be determined arbitrarily depending on the composition of the metallayers 12, etc. Although no special limitation is imposed thereon, ingeneral, X2 is 0.1 to 100 μm, preferably 0.5 to 10 μm, and morepreferably 1 to 5 μm, and Y2 is 0.05 to 100 μm, preferably 0.1 to 10 μm,and more preferably 0.5 to 5 μm. It is preferable that X2 and Y2 bewithin the above range and satisfy the above ratio (Y2/X2).

On the other hand, when the above ratio (Y2/X2) is smaller than 0.1, thestress relaxing effect of the thin metal layers 12 e is too large, andthe thin metal layers 12 e cause a large deformation every time theelement is driven. As a result, there may arise breakage due to metalfatigue, and cracks may occur in the interface with the externalelectrodes 15, so that durability might be deteriorated. Above 0.9, thestress relaxing effect of the thin metal layers 12 e might be lowered,and there might appear a stress concentration point in the element, thuslowering the durability of the element.

Alternatively, a thickness ratio (Z2/X2) is in the range of 1.05 to 2,preferably 1.05 to 1.5, and more preferably 1.1 to 1.2, where X2 is athickness of other metal layer except for the thin metal layer 12 e andthe thick metal layer 12 f (namely, the main metal layer 12 d), and Z2is a thickness of the thick metal layer 12 f. This enables the stressrelaxing effect of the thick metal layers 12 f to be produced moresurely, and also enables the element shape to be retained. Especially,when the above ratio (Z2/X2) is 1.05 to 1.5, the piezoelectric layers 11adjacent to the thick metal layers 12 f, and the piezoelectric layers 11adjacent to the main metal layers 12 d are also drivingly displacedalmost similarly, thereby obtaining the multilayer piezoelectric elementhaving high durability. Alternatively, when the above ratio (Z2/X2) is1.1 to 1.2, this achieves the multilayer piezoelectric element having alarger displacement and high durability. Like X2 and Y2 in theabovementioned ratio (Y2/X2), the specific values of X2 and Z2 may bedetermined arbitrarily depending on the composition of the metal layers12, etc. Although no special limitation is imposed thereon, in general,X2 is 0.1 to 10 μm, preferably 0.5 to 10 μm, and more preferably 1 to 5μm, and Z2 is 0.1 to 200 μm, preferably 0.5 to 15 μm, and morepreferably 1 to 10 μm.

On the other hand, when the above ratio (Z2/X2) is larger than 2, stressmay concentrate on the thick metal layers 12 f, and the interfacebetween the thick metal layer 12 f and the piezoelectric layer 11 mayflake off, so that delamination might occur in the stacked body. Below1.05, the stress concentration effect of the thick metal layers 12 fmight be lowered, and there might appear a stress concentration point inthe element, thus lowering the durability of the element.

Preferably, the main metal layers 12 d have substantially the samethickness. This enables a larger displacement, high responsibility andimproved durability. Preferably, the thickness within a layer of thethin metal layers 12 e and the thick metal layers 12 f is substantiallythe same. The reason for this is as follows. That is, if in the metallayers to which voltages of different polarity are applied, theirrespective thicknesses per layer are substantially the same in the areaof the metal layers overlapped with each other through the piezoelectriclayer 11, it is possible to suppress the resonance phenomena to begenerated when the displacements as the dimensional changes of theelements become identical.

On the other hand, if there is a locally thin portion in the thin metallayers 12 e, stress may concentrate at the locally thin portion whenthin metal layers 12 e deform for relaxing the stress during the timethe element is drivingly deformed. Undesirably, abnormal heat generationmay occur in continuous use. Similarly, if there are a locally thickportion and a locally thin portion in the thick metal layers 12 f,stress may concentrate at the locally thick portion and the locally thinportion during the time the element is drivingly deformed. Undesirably,abnormal heat generation may occur in continuous use.

Here, the thickness of each of the metal layers 12 (the metal layers 12d to 12 f) is a measured value of a surface obtained by cutting themultilayer piezoelectric element in the stacking direction.Specifically, when the metal layers 12 on the cut surface are observedwith a scanning electron microscope (SEM) and a metal microscope, it canbe seen that the metal layers 12 are composed not only metal componentsbut also elements other than metal, such as voids and ceramiccomposition, etc. Therefore, in the cross section of an arbitrary metallayer, arbitrary five locations in a layer composed mainly of metal areselected, and the respective thicknesses capable of being sandwichedbetween arbitrary two parallel lines are measured. The average value ofthese measured values is taken as a metal layer thickness. In thismanner, the thicknesses of the metal layers 12 d, the thick metal layers12 e and the thick metal layers 12 f are measured, thereby obtaining thethickness of each of the metal layers 12 d to 12 f.

In the present embodiment, it is preferable that the thick metal layer12 f having a larger thickness than the main metal layer 12 d and thethin metal layer 12 e having a smaller thickness than the main metallayer 12 d be oppositely disposed with the piezoelectric layer 11 inbetween, as shown in FIG. 10. Thus, the stress during the time theelement is driven can be concentrated on the thick metal layer 12 f soas to disperse the stress exerted on the element. Further, with thearrangement that the thin metal layer 12 e as the stress relaxing layeris adjacent to the metal layer that collects stress, the stress exertedon the element can be dispersedly relaxed.

Especially, the oppositely disposed metal layers adjacent to each otherin the stacking direction with respect to the thin metal layer 12 e arepreferably the thick metal layers 12 f, as shown in FIG. 11. Thus, thestress during the time the element is driven can be concentrated on thethick metal layer 12 f so as to disperse the stress exerted on theelement. Further, with the arrangement that the thin metal layer 12 e asthe stress relaxing layer is adjacent to the metal layer that collectsstress, the stress exerted on the element can be dispersedly relaxed. Bysandwiching the thin metal layer 12 e as the stress relaxing layerbetween the thick metal layers 12 f as the stress collecting layer,stress can be confined within the thin metal layer 12 e, and the stressexerted on the entire element can be dispersedly relaxed. Hence, theapplication of the element to a piezoelectric actuator can provide thepiezoelectric actuator having excellent durability and high reliability.Since a smaller layer number of the sandwiched thin metal layers 12 ecan increase stress confining effect, the optimum layer number is one.

Preferably, the thin metal layer 12 e, the thick metal layer 12 f andthe main metal layer 12 d are arranged in this order with thepiezoelectric layer 11 in between, and in the stacking direction of thestacked body 13, and the main metal layers 12 d are stacked in thedescending order of the thickness thereof. With this arrangement, thestress during the time the element is driven can be concentrated on thethick metal layer 12 f, thereby dispersing the stress exerted on theelement. Further, with the arrangement that the thin metal layer 12 e asthe stress relaxing layer is adjacent to the metal layer that collectsstress, the stress exerted on the element can be dispersedly relaxed.Furthermore, with the arrangement that the main metal layers 12 d aredisposed in the descending order of the thickness thereof, the stresscollected at the thick metal layer 12 f can be dispersed gradually, andthe increased thickness provides a larger displacement of the adjacentpiezoelectric layer 11. These achieve the multilayer piezoelectricelement having a large displacement, excellent durability and highreliability.

If the thicknesses of a plurality of the metal layers 12 are compared,the thick metal layer 12 f preferably has a peak thickness, and there ispreferably a tilted region where the thickness is gradually lowered fromthe thick metal layer 12 f, throughout over two layers, preferably 2 to5 layers in the stacking direction. Thus, the stress during the time theelement is driven concentrates on the thick metal layers 12 f. However,the presence of a predetermined tilted region enables the stresscollected at the thick metal layer 12 f to be dispersed gradually.

Preferably, the metal layers 12 have predetermined voids 12 d′, 12 e′and 12 f′, as shown in FIG. 12. Particularly, the main metal layer 12 dis provided with voids 12 d′, and the area ratio (the void ratio) of thevoids 12 d′ to the entire cross-sectional area in the cross section ofthe metal layer 12 d is 5 to 70%, preferably 7 to 70%, and morepreferably 10 to 60%. This permits a large displacement, therebyobtaining the multilayer piezoelectric element having excellentdisplacement. Especially, when the void ratio is 7 to 70%, or 10 to 60%,the piezoelectric layers 11 can be more smoothly deformed, and thedisplacement of the multilayer piezoelectric element can be increased bythe sufficient electric conductivity of the metal layers 12.

On the other hand, if the void ratio is smaller than 5%, thepiezoelectric layers 11 are constrained by the metal layers 12 when thepiezoelectric layers 11 are deformed by the applied voltage, therebysuppressing the deformation of the piezoelectric layers 11. This reducesthe amount of deformation of the multilayer piezoelectric element, andincreases the internal stress to be generated. As a result, durabilitymight be affected. On the other hand, when the void ratio is larger than70%, extremely narrow portions may occur at the electrode portions.Therefore, the strength of the metal layers 12 themselves may belowered, and cracks are liable to occur in the metal layers 12.Undesirably, disconnection might occur.

Especially, when the thin metal layers 12 e are composed mainly of metaland voids, the multilayer piezoelectric element can have still higherdurability. That is, as shown in FIG. 12, the thin metal layer 12 e ispreferably composed of a plurality of metal parts spaced apart withvoids 12 e′ in between. Thus, when the piezoelectric layers 11 connectedto the thin metal layer 12 e are connected to the portions not filledwith metal, such as the voids 12 e′, in the metal layers, thepiezoelectric body located at that portions causes no displacement evenif a voltage is applied to the element, and causes deformation whenstress is exerted during driving, thereby relaxing the stress (stressrelaxing effect). That is, the thin metal layer 12 e composed of themetal parts functions as a stress relaxing layer. Accordingly, thepiezoelectric layers 11 connected to these metal layers have a smalldriving displacement, thereby avoiding that the stress exerted on theelement concentrates at a point. This achieves the multilayerpiezoelectric element having excellent durability and high reliability.

Specifically, the area ratio (the void ratio) of the voids 12 e′ to theentire cross-section area in the cross section of the thin metal layer12 e is preferably 20 to 90%. This further increases displacement,achieving the multilayer piezoelectric element having excellentdisplacement.

Also in the present embodiment, the metal layers 12 are preferablycomposed mainly of metal selected from elements in groups 8 to 11 of theperiodic table. It is further preferable to compose mainly of metalsatisfying the following relationship of: 0<M1≦15, 85≦M2<100, M1+M2=100,where M1 (% by mass) is a content of an element in the groups 8 to 10 ofthe periodic table, and M2 (% by mass) is a content of an element in thegroup 11 of the periodic table in the metal layer 12.

Particularly when the thin metal layers 12 e relax stress, relaxing theapplied stress means to release the stress by converting the appliedkinetic energy to thermal energy, and the stress releasing portionretains heat. As the temperature of the piezoelectric body is raised,the force of piezoelectric displacement is reduced. Once the temperatureis raised to Curie point, polarization effect will be vanished even ifcooled, and the force of piezoelectric displacement is greatly impaired.Consequently, if the thin metal layers 12 e can play the role of a heatsink, it will become possible to dissipate heat from the stress relaxingportions to the outside of the element.

Here, the use of metal having the composition of the present embodimentincreases the heat dissipation effect, enabling the stress relaxingeffect to be retained with high durability for a long period of time.Particularly, the composition containing a high concentration of silverhaving high thermal conduction can produce the highest thermaldissipation effect. Further, even if oxidized, the thermal conductivitywill not be deteriorated, and electric conductivity will not also bedeteriorated, permitting the stress relaxing layer with extremely highdurability.

The configuration is otherwise similar to those described in the firstto fourth preferred embodiments, and therefore the description thereofis omitted.

(Sixth Preferred Embodiment)

A sixth preferred embodiment related to a multilayer piezoelectricelement of the present invention will next be described with referenceto the drawing. FIG. 13 is a partially enlarged cross section showingthe stacked structure of a multilayer piezoelectric element according tothe present embodiment. In FIG. 13, similar or equivalent parts to theconfigurations of FIGS. 1 to 12 as described above have similarreference numbers, and the description thereof is omitted. As shown inFIG. 13, like the above-mentioned first preferred embodiments, themultilayer piezoelectric element of the sixth preferred embodiment isthe multilayer piezoelectric element in which a plurality ofpiezoelectric layers 11 and a plurality of metal layers 12 are stackedalternately.

A plurality of the metal layers 12 includes a plurality of thick metallayers 12 f having a larger thickness than oppositely disposed metallayers (metal layers 12 d) adjacent to each other in the stackingdirection. This configuration also produces the same effect as theabovementioned preferred embodiments, because the piezoelectric layers11 around the thick metal layers 12 f have a large displacement, and thepiezoelectric layers 11 around the main metal layers 12 d, having asmaller thickness than the thick metal layers 12 f, have a smalldisplacement. This provides the configuration where the metal layershaving different displacements are arranged in the element.

Like the fifth preferred embodiment as described above, a plurality ofthe thick metal layers 12 f of the present embodiment are preferablydisposed interposing in between a plurality of different metal layersother than the thick metal layers 12 f (namely, the main metal layers 12a and the thin metal layers 12 e). Preferably, a plurality of the thickmetal layers 12 f are regularly arranged in the stacking direction.Preferably, a plurality of the metal layers 12 include a plurality ofthin metal layers 12 e having a smaller thickness than oppositelydisposed metal layers adjacent to each other in the stacking direction.

The configuration is otherwise similar to those described in the firstto fifth preferred embodiments, and therefore the description thereof isomitted.

(Seventh Preferred Embodiment)

A seventh preferred embodiment related to a multilayer piezoelectricelement of the present invention will next be described. The multilayerpiezoelectric element of the present embodiment is one in which aplurality of piezoelectric layers 11 and a plurality of metal layers 12are stacked alternately, inactive layers 14 composed of a piezoelectricbody are formed at both sides in the stacking direction, respectively,and the metal layers 11 adjacent to the inactive layers 14 are thinmetal layers (thin metal layers 12 e) having a smaller thickness thanthe metal layers 11 adjacent to each other in the stacking direction.This avoids that the stress exerted on the element concentrates at apoint. The reason for this seems to be as follows.

That is, the inactive layers not sandwiched with electrodes will not bedrivingly deformed even if a voltage is applied. Therefore, a drivinglydeformed portion and a non-drivingly deformed portion are bounded by themetal layer 12 adjacent to the inactive layer 14. At this time, if allof the metal layers 12 have the same thickness, stress may concentrateat a point in the above boundary portion. Therefore, delamination mightoccur when the multilayer piezoelectric element is continuously drivenunder high voltage and high pressure for a long period of time.

When the metal layers 11 adjacent to the inactive layer 14 are the thinmetal layers (the thin metal layers 12 e) having a smaller thicknessthan the metal layers 11 adjacent to each other in the stackingdirection, the thin metal layers 12 e have greater flexibility thanother metal layers. Thus, when the element is driven and thepiezoelectric layers 11 are deformed, the thin metal layers 12 ethemselves can be deformed thereby to relax the stress (stress relaxingeffect). Further, because the inactive layers 14 connected to the thinmetal layers 12 e are formed by a piezoelectric material, the inactivelayers 14 are deformed under stress application, thereby relaxing thestress. That is, the thin metal layers 12 e and the inactive layers 14produce synergism of the stress relaxing effect. Additionally, becausethe thin metal layers 12 e deform themselves, the piezoelectric layer 11sandwiched between the thin metal layer 12 e and the metal layer 12adjacent thereto are subjected to both of the driving deformation due tovoltage application, and the deformation due to stress application.Since the thin metal layers 12 e deform themselves for relaxing stress,the deformation due to stress application is dominant, therebypermitting deformation for relaxing the stress. Consequently, thedriving displacement becomes small, thereby avoiding that the stressexerted on the element concentrates at a point.

Preferably, the metal layer adjacent to the thin metal layer 12 e in thestacking direction is the thick metal layer 12 f. Thereby, the stressduring the time the element is driven can be concentrated on the thickmetal layer 12 f, and the stress exerted on the element can be dispersedinto the ends. Further, by arranging the thin metal layer 12 e servingas a stress relaxing layer so as to be adjacent to the metal layer thatcollects stress, the stress exerted on the element can be dispersedlyrelaxed into the ends. When the thick metal layer 12 f as a stresscollecting layer, and the inactive layer 14 sandwich in between the thinmetal layer 12 e as a stress relaxing later, the stress can be confinedwithin the thin metal layer 12 e, thereby dispersedly relaxing thestress exerted on the entire element. As a result, when the element isapplied to a piezoelectric actuator, it is possible to provide thepiezoelectric actuator having excellent durability and high reliability.

Conventionally, especially when forming a multilayer piezoelectricelement having a stacking number of less than 50, for example, the metalcontent of the piezoelectric layer 11 in the vicinity of the inactivelayer 14 is increased as it approaches the inactive layer 14, in orderthat displacement can be suppressed so as to suppress the stress fromconcentrating at the boundary portion. For this, in order to form thepiezoelectric layer 11, piezoelectric sheets having several kinds ofthickness have to be prepared and stacked one upon another, resulting ina high cost product. On the other hand, in the present invention, themultilayer piezoelectric element having high durability can bemanufactured at a low lost, only by changing the metal layers 12 intothe thin metal layers (the thin metal layers 12 e) having a smallerthickness than the metal layers 12 adjacent to each other in thestacking direction. Further, a multilayer piezoelectric element havinghigh durability can be manufactured at a lower cost by changing themetal layers 12 adjacent to the inactive layers 14 at both sides intothe thin metal layer (the thin metal layer 12 e) having a smallerthickness than the metal layers 12 adjacent to each other in thestacking direction.

On the other hand, in a multilayer piezoelectric element having a largestacking number, by including a plurality of thin metal layers 12 ehaving a smaller thickness than oppositely disposed metal layers (themetal layers 12 d) adjacent to each other in the stacking direction, thepiezoelectric layers around the thin metal layers 12 e have a smalldisplacement because thin metal layers can be easily deformed to absorbthe local stress of piezoelectric displacement. Hence, the metal layershaving different displacements can be separately arranged in theelement. Thus, even if continuously driven under high voltage and highpressure for a long period of time, the suppression of the elementdeformation due to stress concentration can be relaxed, suppressingdelamination to be generated at the stacking portions. In addition,because resonance phenomena can be suppressed, beat sound generation canbe prevented. Furthermore, harmonic signal generation can be prevented,suppressing the noise of control signal.

The configuration is otherwise similar to those described in the firstto sixth preferred embodiments, and therefore the description thereof isomitted.

(Eighth Preferred Embodiment)

An eighth preferred embodiment related to a multilayer piezoelectricelement of the present invention will be described below. The multilayerpiezoelectric element of the present embodiment is one in which aplurality of piezoelectric layers 11 and a plurality of metal layers 12are stacked alternately, and inactive layers 14 composed of apiezoelectric body are formed on both sides in the stacking direction,respectively, and the metal layers 11 adjacent to the inactive layers 14are metal layers (thick metal layers 12 f) having a larger thicknessthan the metal layers 11 adjacent to each other in the stackingdirection. This realizes the multilayer piezoelectric element havingexcellent durability and high reliability. The reason for this seems tobe as follows.

That is, because the inactive layers not sandwiched with electrodes willnot be drivingly deformed even if a voltage is applied, a drivinglydeformed portion and a non-drivingly deformed portion are bounded by themetal layers 12 adjacent to the inactive layers 14, and therefore stressmay concentrate at the boundary portion. At this time, if all of themetal layers 12 have the same thickness, stress may concentrate at apoint in the above boundary portion. Therefore, delamination might occurwhen the multilayer piezoelectric element is continuously driven underhigh voltage and high pressure for a long period of time.

In a state where the thickness of the metal layer 12 adjacent to theinactive layer 14 becomes higher than the thickness of each of the metallayers 12 adjacent to each other in the stacking direction (thick metallayer 12 f), when the element is driven and the piezoelectric layers 11are deformed, the thick metal layers 12 f repel any local stress ofpiezoelectric displacement without their deformation, because theyexhibit strong force constraining not only the inactive layers 14connected to the thick metal layers 12 f, but also the piezoelectriclayers 11 connected to the thick metal layers 12 f. Therefore, thepiezoelectric layers 11 connected to the thick metal layers 12 f cause alarger displacement. This permits an increase in the piezoelectricdisplacement of the element.

Further, when the element is driven, the thick metal layers 12 f are notdeformed themselves for the above reason, and therefore the stressexerted on the entire element concentrates in the vicinity of the thickmetal layers 12 f (stress concentration effect). Thus, by arrangingthese thick metal layers 12 f at the ends of the driving portion of theelement, the stress can be dispersed into the ends of the element,without stress concentration into the driving portion of the element,thus achieving the multilayer piezoelectric element having excellentdurability and high reliability.

Conventionally, especially when forming a multilayer piezoelectricelement having a stacking number of less than 50, for example, the layerthickness of the piezoelectric layer 11 in the vicinity of the inactivelayer 14 is increased as it approaches the inactive layer 14, in orderthat displacement can be suppressed so as to suppress the stress fromconcentrating at the boundary portion. For this, in order to form thepiezoelectric layer 11, several types of piezoelectric sheets have to beprepared and stacked one upon another, resulting in a high cost product.On the other hand, in the present invention, the multilayerpiezoelectric element having high driving force and high durability canbe manufactured at a low lost, only by changing the metal layer 12 intothe thick metal layer (the thick metal layer 12 f) having a largerthickness than each of the metal layers 12 adjacent to each other in thestacking direction. Further, a multilayer piezoelectric element havinghigh durability can be manufactured at a lower cost by changing themetal layers 12 adjacent to the inactive layers 14 at both sides intothe thin metal layer (the thin metal layer 12 e) having a smallerthickness than each of the metal layers 12 adjacent to each other in thestacking direction.

The configuration is otherwise similar to those described in the firstto seventh preferred embodiments, and therefore the description thereofis omitted.

A description will next be made of a method of manufacturing themultilayer piezoelectric elements according to the fifth to eighthpreferred embodiments as described above.

Firstly, in the same manner as in the first to fourth preferredembodiments, a plurality of ceramic green sheets serving as thepiezoelectric layers 11 are manufactured.

Subsequently, a conductive paste is prepared by adding while mixingbinder and plasticizer, etc. in metal powder composing the metal layers12, such as silver-palladium alloy. The conductive paste is then printedin a thickness of 1 to 40 μm on the upper surfaces of the respectivegreen sheets by screen printing or the like.

Here, the thickness of the metal layer 12 can be changed by changing theratio of the binder and the plasticizer to the metal powder, oralternatively by changing the degree of the mesh of a screen used forthe screen printing, or alternatively by changing the thickness of aresist for forming the pattern of the screen. Among others, by changingthe thickness of a resist, the metal layers 12 having differentthicknesses can be formed if a single conductive paste is used.Alternatively, if a single process, a single conductive paste is used,thick metal layer 12 can be formed by stackingly printing at the samelocation.

In order to form voids in the metal layers 12, a conductive paste may beprepared by containing in the above metal powder an organic matter, suchas acryl beads, which are bindingly fixed during drying, and volatizedduring sintering. In order to set the void ratio of the metal layers 12to a predetermined value, there is, for example, a method of changingthe ratio of the above organic matter and the metal powder. That is, theorganic matter may vaporize during sintering, so that voids can beformed in the metal layers 12. Accordingly, a low content of the organicmatter decreases the void ratio, and a high content of the organicmatter increases the void ratio. Specific organic contents of the metallayers 12 d to 12 f are as follows. The metal layer 12 d is 0.1 to 10parts by mass, and preferably 1 to 5 parts by mass with respect to 100parts by mass of metal powder. The thin metal layer 12 e is 0.1 to 50parts by mass, and preferably 2 to 10 parts by mass with respect to 100parts by mass of metal powder. The thick metal layer 12 f is 0.01 to 5parts by mass, and preferably 0.1 to 2 parts by mass with respect to 100parts by mass of metal powder. As the above organic matter, there arethe same organic matters as exemplified in the first to fourth preferredembodiments.

Subsequently, a plurality of the green sheets with the conductive pasteprinted thereon are stacked one upon another. The stacked body with aheavy stone mounted thereon is debindered at a predeterminedtemperature. Thereafter, this is sintered without mounting any heavystone thereon so that the metal layer 12 has a predetermined thickness,thereby obtaining the stacked body 13. The sintering temperature is 900to 1200° C., and preferably 900 to 100° C. The reason for this is asfollows. That is, when the sintering temperature is below 900° C., thesintering temperature is low and the sintering is insufficient, makingit difficult to manufacture a dense piezoelectric body. When thesintering temperature is above 1200° C., there are the followingproblems. That is, the interlayer stress may be increased, which may begenerated in a state in which the piezoelectric layers 11 and the metallayers 12 having different coefficients of thermal expansion areconnected to each other at 1200° C. and above, and then cooled. Thecrystal grains of the piezoelectric body may have abnormal grain growth.The electrode material temperature may be raised over the melting pointthereof, and then melted.

Thereafter, the metal layer 12 whose end is exposed to the side surfaceof the multilayer piezoelectric element, and the metal layer 12 whoseend is not exposed thereto are alternately formed. Then, a groove isformed in a piezoelectric portion between the metal layer 12 whose endis not exposed, and the external electrode 15. An insulator of resin orrubber, having a lower Young's modulus than the piezoelectric layer 11,is formed in the groove. Here, the groove is formed of the side surfaceof the stacked body 13 by using an internal dicing device or the like.

Next, external electrodes 15 are formed in the same manner as in thefirst to fourth preferred embodiments. Silicone rubber is filled intothe groove of the stacked body 13, and silicone rubber is coated on theside surfaces of the stacked body 13 in the same manner as in the firstto fourth preferred embodiments. The silicone rubber, which is filledinto the groove and also coated on the side surfaces of the stacked body13, is then cured, thereby obtaining the multilayer piezoelectricelement.

When this multilayer piezoelectric element is used in a piezoelectricactuator, the polarization processing of the stacked body 13 isperformed by connecting lead wires to the external electrodes 15,respectively, and by applying through the lead wires a dc voltage of 0.1to 3 kV/mm to a pair of the external electrodes 15, respectively. Thisachieves a piezoelectric actuator using the multilayer piezoelectricelement of the present invention. The configuration is otherwise similarto those described in the first to fourth preferred embodiments, andtherefore the description thereof is omitted.

(Ninth Preferred Embodiment)

A ninth preferred embodiment related to a multilayer piezoelectricelement of the present invention will next be described in detail withreference to the accompanying drawings. FIG. 14 is a partially enlargedcross section showing the stacked structure of a multilayerpiezoelectric element according to the present embodiment. In FIG. 14,similar or equivalent parts to the configurations of FIGS. 1 to 13 asdescribed above have similar reference numbers, and the descriptionthereof is omitted.

As shown in FIG. 14, in the multilayer piezoelectric element of thepresent embodiment, a plurality of metal layers 12 are composed mainlyof an alloy, and include a plurality of high-ratio metal layers 12 hhaving a higher ratio of a component constituting the alloy thanoppositely disposed metal layers 12 g adjacent to each other in thestacking direction. That is, because the alloy can change its softness(hardness) freely by its composition, metal layers having partiallydifferent softnesses can be arranged by changing a part of a pluralityof the metal layers 12 into a high-ratio metal layer 12 h. Since thestress exerted on the piezoelectric element can be dispersed, thesuppression of the element deformation due to stress concentration canbe relaxed, thereby increasing the entire displacement of thepiezoelectric element. Additionally, the stress concentration due to theelement deformation can also be suppressed, and it is therefore possibleto suppress the occurrence of delamination in the stacking interface,which can cause breakage, even in a long-term continuous driving underhigh voltage and high pressure.

As described above, the “high-ratio metal layer 12 h” in the presentembodiment is a metal layer having a higher ratio of one componentconstituting the alloy (for example, the ratio of silver constitutingsilver-palladium alloy) than oppositely disposed metal layers 12 gadjacent to each other. A ratio B of one component in the high-ratiometal layer 12 h may be set higher than a ratio A of one component inthe oppositely disposed metal layer 12 g adjacent thereto (B>A). Theratio B is set higher than the ratio A, preferably higher 0.1 and above% by mass, more preferably 0.5 to 10% by mass, and still more preferably1 to 3% by mass. When the ratio B is set higher 0.1% by mass and abovethan the ratio A, it is capable of obtaining the effect of dispersingthe stress exerted on the element. Particularly, when the ratio B is sethigher 0.5% by mass and above than the ratio A, this effect is high. Onthe other hand, when the ratio B is set higher in the range exceeding10% by mass, the coefficient of thermal expansion of the high-ratiometal layer 12 h is different from the coefficient of thermal expansionof the adjacent and oppositely disposed metal layers 12 g. Consequently,the difference in the coefficient of thermal expansion between thepiezoelectric layer and the metal layer contributes to the occurrence ofstress distribution, and a stress concentration point might occur in themultilayer piezoelectric element.

In the multilayer piezoelectric element of the present embodiment, thedrivingly deformed regions correspond to the regions of thepiezoelectric layer 11 where the metal layers 12 disposed on the opposedmain surfaces of the piezoelectric layer 11 are overlapped in thestacking direction with the piezoelectric layer 11 in between.Therefore, in order to obtain the effect of the present embodiment, theratio B of one component in the high-ratio metal layer 12 h and theratio A of one component in the metal layer 12 g may satisfy the aboverelationship in the regions overlapping in the stacking direction withthe piezoelectric layer 11 in between. Thus, the suppression of theelement deformation due to stress concentration can be relaxed, therebyincreasing the entire displacement of the piezoelectric element.Additionally, the stress concentration due to the element deformationcan also be suppressed, and it is therefore possible to suppress thedelamination at the stacking portions even in a long-term continuousdriving under high voltage and high pressure. It is also possible tosuppress resonance phenomena to be generated when the displacements (thedimensional changes) of the piezoelectric elements become identical.This enables prevention of beat sound generation and also prevention ofharmonic signal generation, thereby suppressing the noise of controlsignals. In addition, by arranging a plurality of the high-ratio metallayers 12 h, the magnitude of displacements of the multilayerpiezoelectric element 13 can be controlled, thus eliminating thenecessity to change the thicknesses of the piezoelectric layers 11. Thisprovides the structure that is effective for mass production.

The alloy composition of the metal layers 12 can be measured as follows.That is, a part of the metal layer 12 is taken by, for example, cuttingthe stacked body 13 by the interface between the metal layer 12 and thepiezoelectric layer 11 so as to expose the metal layer 12, followed by achemical analysis, such as ICP (induction coupling plasma) lightemission analysis. Alternatively, the cross-section obtained by cuttingthe multilayer piezoelectric element in the stacking direction may beanalyzed by using EPMA (Electron Probe Micro Analysis) method or thelike. When the metal layer on the cut surface of the multilayerpiezoelectric element is observed with a SEM (scanning electronmicroscope) and a metal microscope, in some cases, not only metalcomponents but also elements other than metal, such as voids and ceramiccomposition, are also contained. In this case, the part consisting onlyof the metal may be analyzed by EPMA method or the like. Thus, the alloyratio of the high-ratio metal layer 12 h and other metal layer 12 g canbe specified.

A plurality of the high-ratio metal layers 12 h are arrangedrespectively, interposing in between one or a plurality of differentmetal layers 12 g other than the high-ratio metal layer 12 h. Forexample, when the alloy constituting the metal layer 12 issilver-palladium and the abovementioned one component is silver, for thefollowing reason, the plurality of the high-ratio metal layers 12 h arepreferably arranged, respectively, interposing in between a plurality ofthe different metal layers 12 g other than the high-ratio metal layers12 h. That is, when the high-ratio metal layers 12 h and the differentmetal layer 12 g are alternately and continuously stacked one by one,there is the merit that the stress in the inside of the multilayerpiezoelectric element 13 is uniformly dispersed to all of the metallayers 12. On the other hand, the high-ratio metal layer 12 h has ahigher silver ratio than the different metal layer 12 g, and hence thehigh-ratio metal layer 12 h itself is soft. Therefore, if the layernumber of the existing high-ratio metal layers 12 h is substantially thesame as the metal layers 12 g, the driving displacement relaxationaction is also enhanced, and driving displacement tends to decrease.Accordingly, by arranging a plurality of the high-ratio metal layers 12h so as to interpose in between a plurality of the different metallayers 12 g, piezoelectric displacement can be increased at thelocations in which a plurality of the different metal layers 12 g areinterposed. Further, the stress relaxing effect can be obtained at thelocations of the plurality of the high-ratio metal layers 12 h. Thisincreases the entire element displacement and also suppresses the stressconcentration due to the element deformation, thereby eliminating thepossibility of flaking of the stacked portions even in a long-termcontinuous driving under high voltage and high pressure.

The alloy constituting the metal layers 12 is composed mainly of metalin groups 8 to 10 and/or metal in group 11 of the periodic table. Thispermits simultaneous sintering of the piezoelectric body and the metallayers, so that the connecting interfaces can be bound firmly. Inaddition, if the element is deformed thereby to apply stress onto themetal layers, because the metal layers can stretch themselves, thestress cannot concentrate at a point, thereby providing a piezoelectricactuator having excellent durability and high reliability.

It is particularly preferable that the alloy constituting the metallayers 12 is silver-palladium alloy, and the abovementioned onecomponent is silver. The reason for this is as follows. That is, themultilayer piezoelectric element 13 can be obtained by sintering in anoxidation atmosphere. Further, because silver and palladium arecompletely solid-dissolved metals, a soft high-ratio metal layer 12 hhaving stress relaxing effect can be formed throughout the metal layersurface, without forming any unstable inter-metal compound.Particularly, by using silver as the metal of the high-ratiocomposition, silver can be solid-dissolved in the liquid phasecomposition of ceramics when sintering the multilayer piezoelectricelement, and the liquid phase forming temperature can be lowered toadvance the sintering. This achieves strong mutually binding forcebetween the metal layer 12 and the piezoelectric layer 11. Furthermore,the alloyzation enables formation of the metal layers 12 having strongermigration resistance than a single element, achieving the multilayerpiezoelectric element with durability.

Preferably, the plurality of the high-ratio metal layers 12 h areregularly arranged. With the irregular arrangement, the stress exertedon the entire multilayer piezoelectric element may concentrate at alocation where the spacing between the high-ratio metal layers is large,and there is a likelihood that sufficient stress dispersing effectcannot be obtained. The regular arrangement of the high-ratio metallayers 12 h enables effective dispersion of the stress exerted on themultilayer piezoelectric element. In the present embodiment, theexpression that “the high-ratio metal layers are regularly arranged”includes the case where the layer number of the different metal layers12 g, which are present between the high-ratio metal layers 12 h, isidentical for each area between the high-ratio metal layers 12 h, aswell as the case where the layer number of the different metal layers 12g existing between the high-ratio metal layers 12 h approaches such adegree that the stress will not concentrate at a portion. Specifically,the layer number of the different metal layers 12 g existing between thehigh-ratio metal layers 12 h is preferably within ±20% with respect tothe average value of the respective layer numbers, more preferablywithin ±10% with respect to the average value of the respective layernumbers, and still more preferably all be identical number. Setting thelayer number of the different metal layers 12 g existing between thehigh-ratio metal layers 12 h to the abovementioned range, the stressexerted on the multilayer piezoelectric element can be dispersed moreeffectively.

The adhesion between the high-ratio metal layer 12 h and thepiezoelectric layer 11 is preferably set lower than the adhesion betweenthe different metal layer 12 g other than the high-ratio metal layer 12h and the piezoelectric layer 11. Thus, in a state in which thehigh-ratio metal layer 12 h has lower adhesion than the different metallayer 12 g, when stress is exerted on the multilayer piezoelectricelement, the high-ratio metal layer 12 h, whose adhesion is weak, isdeformed to relax the stress. In the piezoelectric layer 11 connected tothe high-ratio metal layer 12 h of weak adhesion, the contact area withthe high-ratio metal layer 12 h is reduced, thereby decreasing the forceconstraining the piezoelectric layer 11. This also enables to relax thestress exerted on the multilayer piezoelectric element, and avoid stressconcentration at a point, achieving the multilayer piezoelectric elementhaving excellent durability.

Preferably, the Vickers hardness of the high-ratio metal layer 12 h isset lower than that of the different metal layer 12 g. By setting theVickers hardness of the high-ratio metal layer 12 h to be lower thanthat of the different metal layer 12 g, namely, by setting it to be ametal layer softener than the different metal layer 12 g, when thepiezoelectric element is driven, the high-ratio metal layer 12 h becomesweak in the force constraining the piezoelectric layer 11 connected tothe high-ratio metal layer 12 h, enabling the piezoelectric layer 11 tocause a large displacement. This permits the multilayer piezoelectricelement having high durability and a large displacement.

In the multilayer piezoelectric element according to the presentembodiment, because the metal layer 12 has a small thickness in thestacking direction, the Vickers hardness of the metal layer 12 can bemeasured as follows. That is, the Vickers hardness is measured by using,for example, a Micro Vickers Tester such as Model MVK-H3 manufactured byAkashi Seisakusho Co., Ltd. The Vickers hardness of the metal layer 12can also be measured by cutting the multilayer piezoelectric element inthe vicinity of the interface between the metal layer 12 and thepiezoelectric layer 11, and forcing a diamond probe into the metal layer12. In order to avoid the influence of the piezoelectric layer 11 as abase, it is preferable to force the diamond probe into the metal layer12 from a direction perpendicular to the stacking direction. When themetal layer 12 is exposed from the side surface of the piezoelectricelement, the multilayer piezoelectric element is placed so that thediamond probe and the stacking direction of the metal layer 12 areperpendicular to each other, and the hardness is measured by directlyforcing the diamond probe into the metal layer 12.

On the other hand, when the metal layer 12 is not exposed from the sidesurface of the piezoelectric element, the element is polished until themetal layer 12 is exposed, and then the hardness is measured in the samemanner as described above. In order to expose the metal layer 12,instead of polishing, it may be cut with a dicing saw machine, oralternatively a luter may be used. No limitation is imposed on thetechnique as long as a flat surface can be formed without causing anycracks or the like.

A plurality of the different metal layers other than the high-ratiometal layer 12 h are disposed between two pieces of the high-ratio metallayers 12 h, and a tilted concentration part where the concentration ofone component constituting the alloy is gradually reduced from thehigh-ratio metal layer side is present in a group of the different metallayers. By the presence of the tilted concentration part, the stressexerted on the multilayer piezoelectric element can be concentrated onthe high-ratio metal layers 12 h, and the stress can also be dispersedinto the metal layer 12 g (the metal layer 12 g in the tiltedconcentration part) in the vicinity of the high-ratio metal layer 12 h,thereby permitting the multilayer piezoelectric element having higherdurability. The presence of the tilted concentration part in every areabetween the high-ratio metal layers 12 h is more preferable for furtherenhancing durability.

On the other hand, when the high-ratio metal layer 12 h and the metallayer 12 g adjacent thereto have extremely different ratios of onecomponent constituting the alloy, there is a likelihood that stress isliable to concentrate on the high-ratio metal layer 12 h serving as thestress relaxing layer.

Like the foregoing preferred embodiments, the metal layers 12 preferablyhave a large number of voids in the present embodiment. Particularly, itis preferable that the different metal layer 12 g other than thehigh-ratio metal layer is provided with voids, and the area ratio of thevoids to the entire cross-sectional area in the cross section of themetal layer 12 is 5 to 70%. This is because the multilayer piezoelectricelement having a larger displacement and excellent displacement propertycan be obtained by the presence of the voids constituting 5 to 70% ofthe area of the different metal layer 12 g other than the high-ratiometal layer.

On the other hand, if the void ratio of the different metal layer 12 gis smaller than 5%, the piezoelectric layers 11 are subjected to a largeforce of constraint from the metal layer 12 g when the piezoelectriclayers 11 are deformed by the applied voltage, thereby suppressing thedeformation of the piezoelectric layers 11. Accordingly, there is thelikelihood that the amount of deformation of the multilayerpiezoelectric element is reduced, and the internal stress to begenerated is also increased. On the other hand, when the void ratio islarger than 70%, extremely narrow portions may occur at the electrodeportions. Therefore, the strength of the metal layers themselves may belowered, so that cracks are liable to occur in the metal layers 12 g,and disconnection might occur. The void ratio is more preferably 7 to70%, and more preferably 10 to 60%. By so doing, the piezoelectriclayers 11 can be more smoothly deformed, and the displacement of themultilayer piezoelectric element can be increased by the presence ofsufficient electric conductivity of the metal layers 12.

It is also preferable that the area ratio of the voids to the entirecross-section area in the cross section of the high-ratio metal layer 12h is 20 to 90%. This is because the multilayer piezoelectric elementhaving a larger displacement and excellent displacement property can beobtained by the presence of the voids constituting 20 to 90% of the areaof the high-ratio metal layer 12 h.

Further, the metal layers 12 composed mainly of metal and voids enableachievement of the multilayer piezoelectric element with higherdurability, because both of the metal and the voids are deformableagainst stress. Particularly, when the high-ratio metal layer 12 hrather than the different metal layer 12 g other than the high-ratiometal layer is composed mainly of metal and voids, because both of themetal and the voids are deformable against stress, the stress relaxingeffect can be improved, permitting the multilayer piezoelectric elementwith higher durability.

Preferably, the high-ratio metal layer 12 h takes the form that aplurality of alloys are scattered. That is, the high-ratio metal layer12 h is preferably formed by a plurality of conductive regions scatteredin the shape of islands. By the plurality of the conductive regionsscattered in the high-ratio metal layer 12 h, even if the stress exertedon the multilayer piezoelectric element 13 is propagated to the metallayers 12, the stress propagation within the high-ratio metal layer 12 hcan be suppressed, causing no particular location where the stress isconcentrated in the high-ratio metal layer 12. Consequently, stressrelaxation and durability are compatible.

On the other hand, in cases where the high-ratio metal layer 12 h iscomposed of a single continuous layer, when the stress exerted on themultilayer piezoelectric element 13 concentrates on the high-ratio metallayer 12 h, the stress will propagate and concentrate at the portion ofthe interface with the piezoelectric layer 11 which is faced to the sidesurface of the piezoelectric element. Hence, there is the likelihood ofoccurrence of the location at which the stress particularlyconcentrates.

Also in the present embodiment, the main component is preferably a metalcomposition satisfying the following relationships of: 0<M1≦15,85≦M2<100, M1+M2=100, where M1 (% by mass) is a palladium content, andM2 (% by mass) is a silver content in the metal layer 12.

A description will next be made of a method of manufacturing themultilayer piezoelectric elements according to the ninth preferredembodiment.

Firstly, in the same manner as in the first to eighth preferredembodiments, ceramic green sheets serving as the piezoelectric layers 11are manufactured. Subsequently, a conductive paste is prepared by addingwhile mixing binder and plasticizer, etc. in metal powder composing themetal layers 12, such as silver-palladium alloy. The conductive paste isthen printed in a thickness of 1 to 40 μm on the upper surfaces of therespective green sheets by screen printing or the like.

In the conductive paste for forming the high-ratio metal layer 12 h, theamount of one component in metal powder contained in the conductivepaste is set larger than the amount of one component contained in theconductive paste for forming the different metal layer 12 g.Specifically, when silver-palladium as an alloy is used to increase thesilver component of the high-ratio metal layer 12 h, a metal pastecontaining much silver component in the alloy composition is used toform the high-ratio metal layer 12 h, and a metal paste containing lesssilver component in the alloy composition is used to form the differentmetal layer 12 g other than high-ratio metal layer. Instead of the alloypowder, a mixed powder of silver powder and palladium powder may be usedto adjust the composition. Alternatively, silver powder or palladiumpowder may be added into the silver-palladium alloy in order to adjustthe composition. It is however preferable to add initially alloy powdershaving different compositions, because the metal dispersion within thepaste becomes uniform, and the composition distribution within the samesurface of the metal layer 12 becomes uniform.

Subsequently, a plurality of the green sheets with the conductive pasteprinted thereon are stacked one upon another, and debindered at apredetermined temperature. Thereafter, this is sintered at 900 to 1200°C., thereby manufacturing the stacked body 13. The inactive layers 14may be formed in the same manner as in the foregoing first to eightpreferred embodiments.

Next, external electrodes 15 are formed in the same manner as in thefirst to eighth preferred embodiments. Silicone rubber is filled intothe groove of the stacked body 13, and silicone rubber is coated on theside surfaces of the stacked body 13 in the same manner as in the firstto eighth preferred embodiments. Then, in the stacked body 13 providedwith the external electrodes 15, silicone rubber is filled into thegroove of the stacked body 13 and also coated on the side surfaces ofthe stacked body 13. Thereafter, the silicone rubber, which has beenfilled into the groove and coated on the side surface of the stackedbody 13, is then cured, thereby completing the multilayer piezoelectricelement of the present embodiment.

Finally, the polarization processing of the stacked body 13 is performedby connecting lead wires to the external electrodes 15, respectively,and then applying through the lead wires a dc voltage of 0.1 to 3 kV/mmto the pair of the external electrodes 15, respectively. This results ina piezoelectric actuator using the multilayer piezoelectric element ofthe present invention. The configuration is otherwise similar to thosedescribed in the first to eighth preferred embodiments, and thereforethe description thereof is omitted.

While the ninth preferred embodiment has been described, the multilayerpiezoelectric elements of the present invention are not be limited tothe ninth preferred embodiment but are susceptible of various changeswithout departing from the gist of the present invention. For example,although the ninth preferred embodiment has described the case where allof the metal layers are composed of the alloy, a part of the multilayersmay be composed of an alloy, and the rest may be composed of a singlemetal, as in a tenth preferred embodiment descried later. Although theninth preferred embodiment has described the case where the metal layerscontain the same composition, the metal layers may be composed of atleast two kinds of layers having different main compositions, as in aneleventh preferred embodiment to be described later.

(Tenth Preferred Embodiment)

A tenth preferred embodiment related to a multilayer piezoelectricelement of the present invention will next be described with referenceto the drawing. FIG. 15 is a partially enlarged cross section showingthe stacked structure of a multilayer piezoelectric element according tothe present embodiment. In FIG. 15, similar or equivalent parts to theconfigurations of FIGS. 1 to 14 as described above have similarreference numbers, and the description thereof is omitted.

The multilayer piezoelectric element of the present embodiment has astacked body 13 in which a plurality of piezoelectric layers 11 and aplurality of metal layers 12 (12 i, 12 j) are stacked alternately, andthese metal layers 12 include a plurality of high-ratio metal layers 12j having a higher ratio of at least one component constituting the metallayers 12 than oppositely disposed metal layers 12 i adjacent to eachother in the stacking direction.

By including the plurality of the high-ratio metal layers 12 j, themetal layers having partially different softnesses (hardnesses) can bearranged, thereby dispersing the stress exerted on the piezoelectricelement. Since the suppression of the element deformation due to stressconcentration can be relaxed, the displacement of the entire element canbe increased, and the stress concentration due to the elementdeformation can also be suppressed. This enables suppression of flakingof the stacked portions even in a long-term continuous driving underhigh voltage and high pressure.

The drivingly deformed locations of the piezoelectric layers 11correspond to the locations sandwiched between the metal layers 11.Therefore, it is preferable to form metal layers having different metalcompositions at the portions of the metal layers 12 which are overlappedwith the piezoelectric layer 11 in between. This can suppress resonancephenomena to be generated when the displacements (the dimensionalchanges) of the elements become identical. This also enables preventionof beat sound generation and prevention of harmonic signal generation,thereby suppressing the noise of control signals. In addition, bychanging the metal composition of the metal layers, the magnitude ofdisplacement of the multilayer piezoelectric element 13 can becontrolled, thus eliminating the necessity to change the thicknesses ofthe piezoelectric layers 11. This provides the structure that iseffective for mass production. The metal composition of the metal layerscan be measured by the same method as described above.

Preferably, a plurality of the high-ratio metal layers 12 j are arrangedrespectively, interposing in between a plurality of different metallayers 12 i other than the high-ratio metal layers 12 j. When thehigh-ratio metal layers 12 j and the different metal layer 12 i arealternately and continuously stacked one by one, the stress in theinside of the multilayer piezoelectric element 13 is uniformly dispersedto all of the metal layers 11, while the driving displacement will alsobe relaxed when the multilayer piezoelectric element is driven.Accordingly, by arranging a plurality of the high-ratio metal layers 12j so as to interpose in between a plurality of the different metallayers 12 i, piezoelectric displacement can be increased at thelocations in which a plurality of the different metal layers 12 i areinterposed, thereby permitting the stress relaxation at the locations ofthe plurality of the high-ratio metal layers 12 j. This increases theentire element displacement and also suppresses the stress concentrationdue to the element deformation, thereby suppressing flaking of thestacked portions even in a long-term continuous driving under highvoltage and high pressure.

Specifically, it is preferable that the one component constituting themetal layers 12 be silver, the different metal layers 12 i be composedof silver-palladium alloy, and the high-ratio metal layers 12 j becomposed of silver. The reason for this is as follows. That is, themultilayer piezoelectric element 13 can be obtained by sintering in anoxidation atmosphere. Further, because silver and palladium arecompletely solid-dissolved metals, a soft high-ratio metal layerproducing stress relaxing effect can be formed throughout the metallayer surface, without forming any unstable inter-metal compound.Particularly, by using silver as the metal of the high-ratiocomposition, silver can be solid-dissolved in the liquid phasecomposition of ceramics when sintering the multilayer piezoelectricelement, and the liquid phase forming temperature can be lowered toadvance the sintering. This achieves strong mutually binding forcebetween the metal layer 12 and the piezoelectric layer 11. Furthermore,the alloyzation enables formation of the metal layers having strongermigration resistance than a single element, achieving the multilayerpiezoelectric element with durability.

Thus, under the condition that high-ration metal layers 12 j arecomposed mainly of silver, and the different metal layers 12 i otherthan the high-ratio metal layers are composed mainly of silver-palladiumalloy, the largest stress relaxing effect is attained. When thehigh-ratio metal layers 12 j are adjacent to each other with thepiezoelectric layer 11 in between, the migration of silver might causeinsulation failure. In this case, because the metal layer adjacent tothe high-ratio metal layer 12 j composed mainly of silver is the metallayer 12 i composed mainly of silver-palladium alloy, if silver attemptsto migrate, it combines with palladium and eliminates floating silverions, achieving stabilization. As a result, no insulation failure due tomigration may occur, permitting the multilayer piezoelectric elementhaving high durability.

For the same reason as described in the ninth preferred embodiment, itis preferable that the plurality of the high-ratio metal layers 12 j beregularly arranged, and more preferably, the adhesion between thehigh-ratio metal layer 12 j and the piezoelectric layer 11 is lower thanthe adhesion between the different metal layer 12 i and thepiezoelectric layer 11. It is also preferable that a plurality of thedifferent metal layers 12 i be disposed between the two high-ratio metallayers 12 j, and there be a tilted concentration part where theconcentration of one component is gradually reduced from the high-ratiometal layer 12 j is present in a group of the different metal layers 12i. It is more preferable that the metal layers 12 have a plurality ofvoids, and the high-ratio metal layers 12 j are formed by a plurality ofconductive films scattered in the shape of islands.

Also in the present embodiment, the main component is preferably a metalcomposition satisfying the following relationships of: 0≦M1≦15,85≦M2≦100, M1+M2=100, where M1 (% by mass) is a palladium content, andM2 (% by mass) is a silver content in the metal layer 12 i.

A method of manufacturing the multilayer piezoelectric element accordingto the tenth preferred embodiment may be the same as that in the ninthpreferred embodiment, except that silver powder is added to a conductivepaste for forming the high-ration metal layers 12 j.

The configuration is otherwise similar to those described in the firstto ninth preferred embodiments, and therefore the description thereof isomitted.

(Eleventh Preferred Embodiment)

An eleventh preferred embodiment related to a multilayer piezoelectricelement of the present invention will next be described in detail withreference to the accompanying drawings. FIG. 16 is a partially enlargedcross section showing the stacked structure of a multilayerpiezoelectric element according to the present embodiment. In FIG. 16,similar or equivalent parts to the configurations of FIGS. 1 to 15 asdescribed above have similar reference numbers, and the descriptionthereof is omitted.

The multilayer piezoelectric element of the present embodiment has astacked body 13 in which a plurality of piezoelectric layers 11 and aplurality of metal layers 12 are stacked alternately. These metal layers12 are composed of two kinds of metal layers 12 k and 12 l havingdifferent main components, and a plurality of the metal layers 12 l areinterposed between a plurality of different metal layers 12 k. Thesoftness (hardness) of the metal layers can be changed freely by thecomposition thereof. In the present embodiment, the above arrangement ofthe two kinds of the metal layers 12 k and 12 l having different maincomponents enables the arrangement of the metal layers having partiallydifferent softnesses, thereby dispersing the stress exerted on theelement. This relaxes the suppression of the element deformation due tostress concentration, thereby increasing the displacement of the entirepiezoelectric element. Additionally, the stress concentration due to theelement deformation can also be suppressed, thereby suppressing flakingof the stacked portions even in a long-term continuous driving underhigh voltage and high pressure.

Specifically, it is preferable that the metal layers 12 l be composedmainly of silver-palladium alloy, and the different metal layers 12 k becomposed mainly of copper. This enables the multilayer piezoelectricelement 13 to be configured by sintering in reducing atmosphere such asnitrogen atmosphere. Further, since silver and copper and palladium aresolid-dissolved metals, it becomes the soft metal layers producingstress relaxing effect throughout the metal layer surfaces, withoutforming any unstable inter-metal compound.

Particularly, by using silver-palladium alloy as the main component ofthe metal layers 12 l interposing in between a plurality of thedifferent metal layers 12 k, silver can be solid-dissolved in the liquidphase composition of ceramics when sintering the multilayerpiezoelectric element, and the liquid phase forming temperature can belowered to advance the sintering. This achieves strong mutually bindingforce between the metal layer 12 and the piezoelectric layer 11.Furthermore, the alloyzation enables formation of the metal layershaving stronger migration resistance than a single element, achievingthe multilayer piezoelectric element with durability.

Thus, under the condition that the metal layers 12 l are composed mainlyof silver, and the different metal layers 12 k are composed mainly ofcopper, the largest stress relaxing effect is attained. When the metallayers 12 l composed mainly of silver are adjacent to each other withthe piezoelectric layer 11 in between, the migration of silver mightcause insulation failure. In the present embodiment, because the metallayer adjacent to the metal layer 12 l composed mainly of silver is themetal layer 12 k composed mainly of copper, if silver attempts tomigrate, it combines with copper and eliminates floating silver ions,achieving stabilization. As a result, no insulation failure due tomigration may occur, permitting the multilayer piezoelectric elementhaving high durability.

For the same reason as described in the ninth preferred embodiment, itis preferable that the plurality of the metal layers 12 l be regularlyarranged, and more preferably, the adhesion between the metal layer 12 land the piezoelectric layer 11 be lower than the adhesion between thedifferent metal layer 12 k and the piezoelectric layer 11. It is alsopreferable that a plurality of the different metal layers 12 k bedisposed between the two metal layers 12 l, and a tilted concentrationpart where the concentration of one component is gradually reduced fromthe metal layer 12 l be present in a group of the different metal layers12 k. It is more preferable that the metal layers 12 have a plurality ofvoids. Particularly, it is preferable that the different metal layers 12k have voids, and the area ratio of the voids to the entirecross-sectional area in the cross section of the metal layers 12 k be 5to 70%. This is because the voids constituting 5 to 70% to the area ofthe metal layers 12 k can increase displacement, thereby obtaining themultilayer piezoelectric element having excellent displacement.

If the void ratio of the metal layers 12 k is smaller than 5%, thepiezoelectric layers 11 are constrained by the metal layers when thepiezoelectric layers 11 are deformed by the applied electric field,thereby suppressing the deformation of the piezoelectric layers 11. Thisreduces the amount of deformation of the multilayer piezoelectricelement, and increases the internal stress to be generated. As a result,durability might be affected. On the other hand, if the void ratio ofthe metal layers 12 k is larger than 70%, extremely narrow portions mayoccur at the electrode portions. Therefore, the strength of the metallayers themselves may be lowered, and cracks are liable to occur in themetal layers. Undesirably, disconnection might occur. More preferredvoid ratio is 7 to 70%, and still more preferred is 10 to 60%. By sodoing, the piezoelectric layers 11 can be more smoothly deformed, andthe displacement of the multilayer piezoelectric element can beincreased by the sufficient electric conductivity of the metal layers12.

It is also preferable that the area ratio of the voids to the entirecross-sectional area in the cross section of the metal layers 12 l be 24to 90%. This is because the voids constituting 24 to 90% to the area ofthe metal layers 12 l can further increase displacement, therebyobtaining the multilayer piezoelectric element having excellentdisplacement.

Further, the metal layers 12 composed mainly of metal and voids enableachievement of the multilayer piezoelectric element with higherdurability, because both of the metal and the voids are deformableagainst stress. Particularly, when the metal layer 12 l rather than themetal layer 12 k is composed mainly of metal and voids, because both ofthe metal and the voids are deformable against stress, the stressrelaxing effect can be improved, permitting the multilayer piezoelectricelement with higher durability.

More preferably, the metal layer 12 l takes the form that a plurality ofmetal are scattered. That is, the metal layer 12 l is preferably formedby a plurality of conductive regions scattered in the shape of islands.By the plurality of the conductive regions scattered in the metal layer12 l, even if the stress exerted on the multilayer piezoelectric element13 is propagated to the metal layers 12, the stress propagation withinthe metal layer 12 l can be suppressed, causing no particular locationwhere the stress is concentrated in the metal layer 12 l. Consequently,stress relaxation and durability are compatible.

Also in the present embodiment, the main component is preferably a metalcomposition satisfying the following relationships of: 0≦M1≦15,85≦M2≦100, M1+M2=100, where M1 (% by mass) is a palladium content, andM2 (% by mass) is a silver content in the metal layer 12 l. The reasonfor this is as follows. That is, when the palladium exceeds 15% by mass,specific resistance is increased, and when the multilayer piezoelectricelement is continuously driven, the metal layers 12 generate heat. Theheat generation acts on the piezoelectric layers 11 having temperaturedependency thereby to reduce the displacement characteristic thereof,and in some cases, the element displacement may become small. Further,when the external electrodes 15 are formed, the external electrodes 15and the metal layers 12 are mutually diffused and connected to eachother. However, if the palladium exceeds 15% by mass, this increases thehardness of locations where the metal layer composition is diffused intothe external electrodes 15. Therefore, durability might be lowered inthe multilayer piezoelectric element causing dimensional changes duringdriving.

A method of manufacturing the multilayer piezoelectric element accordingto the eleventh preferred embodiment may be the same as that in theninth preferred embodiment, except that copper powder is added to aconductive paste for forming the different metal layers 12 k. For thepurpose of improving connecting strength between the external electrodes15 and the metal layers 12, it is preferable to use a metal pastecomposing mainly of copper as the metal constituting the externalelectrodes 15. In order to configure the external electrodes 15,irrespective of silver electrodes or copper electrodes, the oxidation ofthe metal layers 12 can be suppressed by performing sintering inreducing atmosphere such as nitrogen atmosphere, thereby achieving themetal layers 12 having high durability.

Although the eleventh preferred embodiment has described the case wherea plurality of metal layers are composed of two kinds of metal layershaving different main components, the effect of the present invention isobtainable as long as there are a plurality of metal layers consistingof at least two kinds of metal layers having different main components,and a plurality of metal layers of one of the two kinds are arrangedinterposing in between a plurality of metal layers of the other type.That is, by configuring so that the stress exerted on the piezoelectricelement is concentrated in the vicinity of the metal layers havingdifferent main metal components, and the collected stress is confined byusing, as a stress relaxing layer, the piezoelectric layers around themetal layers, the collected stress can be confined between the two metallayers having high metal composition. This enables relaxation of thestress exerted on the entire piezoelectric element, thereby providing apiezoelectric actuator having excellent durability and high reliability.

(Twelfth Preferred Embodiment)

A twelfth preferred embodiment related to a multilayer piezoelectricelement of the present invention will next be described. In theconventional multilayer piezoelectric elements, attempts have been madeto form uniform metal layers so that an electric field can be applieduniformly to all of piezoelectric bodies. Particularly, for the purposesof making uniform the electric conductivity of the respective metallayers, and making uniform the surface area of the portions connected tothe piezoelectric bodies, attempts have been made to make uniform themetal filling rates of the metal layers. Therefore, the stress alongwith displacement may concentrate at the outer periphery of the centerin the stacking direction of a multilayer piezoelectric element, causingdisadvantages such as cracks.

Especially, in multilayer piezoelectric elements of simultaneoussintering type, and multilayer piezoelectric elements of the type inwhich at least part of the outer periphery of a piezoelectric body isconstrained, there is a high possibility that stress may concentrate atthe outer periphery of the center of the element when the element iscontinuously driven for a long period of time under high voltage andhigh pressure. This leads to cracks and flaking, and contributes to theproblem of displacement variations.

The present inventors have found the following new fact and completedthe present embodiment. That is, the multilayer piezoelectric elementshaving excellent durability can be obtained by arranging a plurality ofhigh-resistance metal layers having a higher electrical resistance thanoppositely disposed metal layers adjacent to each other, and there is novariation in displacement when the element is continuously driven for along period of time under high voltage and high pressure.

Specifically, the multilayer piezoelectric element according to thepresent embodiment can take the following configurations.

(1) A multilayer piezoelectric element in which a plurality ofpiezoelectric layers and a plurality of metal layers are stackedalternately, in which a plurality of the metal layers include aplurality of high-resistance metal layers having a higher resistancethan oppositely disposed metal layers adjacent to each other in astacking direction.

(2) The multilayer piezoelectric element as set forth in the above (1),in which a plurality of the high-resistance metal layers arerespectively arranged interposing in between a plurality of differentmetal layers other than the high-resistance metal layers.

(3) The multilayer piezoelectric element as set forth in the above (1)or (2), in which the high-resistance metal layers are regularlyarranged.

(4) The multilayer piezoelectric element as set forth in any one of theabove (1) to (3), in which the high-resistance metal layers have ahigher internal void ratio than the different metal layers.

(5) The multilayer piezoelectric element as set forth in any one of theabove (1) to (3), in which the high-resistance metal layers include ahigh resistance component having a higher electrical resistance than thedifferent metal layers, a content of the high resistance component ishigher than a content of the different metal layers.

(6) The multilayer piezoelectric element as set forth in any one of theabove (1) to (5), in which the high-resistance metal layers have asmaller thickness than the different metal layers.

(7) The multilayer piezoelectric element as set forth in any one of theabove (1) to (6), in which an electrical resistance of thehigh-resistance metal layers is 1/10 to 1000 times greater than that ofthe piezoelectric layers.

(8) The multilayer piezoelectric element as set forth in any one of theabove (1) to (7), in which the electrical resistance of thehigh-resistance metal layers is at least 1000 times greater than that ofthe different metal layers.

In accordance with the present embodiment, a plurality of the metallayers include a plurality of high-resistance metal layers having ahigher electrical resistance than adjacent and oppositely disposed metallayers. With the arrangement of a plurality of the high-resistance metallayers, the piezoelectric layers connected to the high-resistance metallayers can have a small displacement. By the presence of a plurality ofthe piezoelectric layers having a small displacement in the multilayerpiezoelectric, stress distribution generated by displacement can bedispersed, thereby suppressing the occurrence of cracks. If cracksoccur, the progress thereof can be suppressed. Consequently, even in along-term continuous driving under high voltage and high pressure, it iscapable of suppressing the change in the desired displacement, therebyproviding the multilayer piezoelectric element having excellentdurability and high reliability.

By using the multilayer piezoelectric element of the present embodiment,it is capable of providing an injector having excellent durability andhigh reliability. That is, the injector contains the multilayerpiezoelectric element as set forth in any of the above (1) to (8), in acontainer having an injection hole.

The multilayer piezoelectric elements according to the presentembodiment will be described in detail with reference to theaccompanying drawings. FIG. 18 is a schematic cross section showing thestacked structure of metal layers connected to piezoelectric layers ofthe multilayer piezoelectric element according to the presentembodiment. In FIG. 18, similar or equivalent parts to theconfigurations of FIGS. 1 to 17 as described above have similarreference numbers, and the description thereof is omitted.

As shown in FIG. 18, in the multilayer piezoelectric element of thepresent embodiment, a plurality of metal layers 12 include a pluralityof high-resistance metal layers 12 m having a higher electricalresistance than adjacent and oppositely disposed metal layers. Thesehigh-resistance metal layers 12 m are arranged interposing in between aplurality of different metal layers 12 n other than the high-resistancemetal layers 12 m. That is, a plurality of the metal layers 12 arecomposed of a plurality of the metal layers 12 n and a plurality ofhigh-resistance metal layers 12 m having a higher electrical resistancethan the metal layers 12 n.

In the conventional multilayer piezoelectric elements, substantiallyuniform metal layers 12 are formed so that an electric field can beapplied uniformly to all of the piezoelectric layers 11. Therefore,during driving, the element itself continuously causes dimensionalchanges, so that all of the piezoelectric bodies 11 are closely drivenwith the metal layer 12 in between. The multilayer piezoelectric elementwill therefore cause driving deformation as a whole. Consequently, thestress due to the element deformation will concentrate at the outerperiphery of the center of the element, which expands at the time ofcompression, and necks at the time of stretch. When the element iscontinuously driven for a long period of time under high voltage andhigh pressure, there has been the problem that the stacked portions (theinterface between the piezoelectric layer and the metal layer) may beflaked, or cracks may occur.

On the other hand, as in the present embodiment, the arrangement of aplurality of the high-resistance metal layers 12 m permits dispersion ofthe stress generated by displacement, thus enabling suppression of theoccurrence of cracks and a reduction in displacement variations even ina long-term continuous driving under high voltage and high pressure,thereby achieving an improvement of durability. The piezoelectric layers11 connected to the high-resistance metal layers 12 m cause lessdisplacement than the piezoelectric layers 11 connected to the differentmetal layers 12 n. This means that there exist a plurality of thepiezoelectric layers 11 having a small displacement, thereby producing astate in which the element is not drivingly deformed as a whole, but aplurality of regions divided by the high-resistance metal layers 12 mare drivingly deformed, respectively. Hence, the element of the presentembodiment enables the stress, which has conventionally beenconcentrated at the center of the element, to be dispersed for each ofthese regions, thereby exhibiting excellent durability even under highvoltage and high pressure. In the event of the partial flaking of thestacked portions or the occurrence of cracks, the piezoelectric layers11 having a small displacement can suppress the progress of the cracks.For the above reason, it is estimated that durability can be improvedthereby to achieve the element having high reliability.

Although a larger number of the high-resistance metal layers 12 mdisperse more stress and achieve more improvement of durability, and toolarge number thereof tends to reduce displacement. Therefore, a suitablenumber thereof is not more than 20% of the total number of thepiezoelectric layers 11.

Preferably, the high-resistance metal layers 12 m are regularly arrangedin the stacking direction of the multilayer piezoelectric element. Thatis, a plurality of the different metal layers 12 n are interposedbetween the two high-resistance metal layer 12 m, so that a plurality ofthe high-resistance metal layers 12 m are substantially regularlyarranged in the stacking direction. This arrangement enables theoccurrence of stress generated by displacement to be substantiallyuniformly dispersed by the portions divided by the high-resistance metallayers 12, respectively. This systematic stress dispersion permitssuppression of the occurrence of cracks and suppression of displacementvariations during driving, thereby improving durability.

In the present embodiment, the expression that “the high-resistancemetal layers are regularly arranged” includes the case where the layernumber of the different metal layers 12 n, which are present between thehigh-resistance metal layers 12 m, is identical for each area betweenthe high-resistance metal layers 12 m, as well as the case where thelayer number of the different metal layers 12 n existing between thehigh-resistance metal layers 12 m approaches such a degree that thestress can be dispersed substantially uniformly in the stackingdirection. Specifically, the layer number of the different metal layers12 n existing between the high-resistance metal layers 12 m ispreferably within ±20% with respect to the average value of therespective layer numbers, more preferably within ±10% with respect tothe average value of the respective layer numbers, and still morepreferably all be identical number.

The void ratio in the high-resistance metal layers 12 m is preferablylarger than the void ratio in the different metal layers 12 n. Under thecondition that the high-resistance metal layers 12 m have a larger voidratio than the different metal layers 12 n, the piezoelectric layer 11connected to the high-resistance metal layer 12 m causes lessdisplacement than the piezoelectric layer 11 whose both main surfacesare connected to the different metal layer 12 n. Thus, the regions, eachof which is divided by the piezoelectric layers 11 having a smalldisplacement, has a smaller displacement than the displacement of theentire multilayer piezoelectric element. These regions are thereforecapable of suppressing cracks generated in the outer periphery of themultilayer piezoelectric element, permitting an improvement ofdurability. Additionally, a large void ratio enables stress absorption,permitting a further improvement of durability.

The porosity (void ratio) of the high-resistance metal layers 12 m ispreferably 40% to 99%, and more preferably 50 to 90%. The reason forthis is as follows. That is, when the void ratio is smaller than 40%,the electrical resistances of the metal layers are not increased, andthere is the likelihood that the displacement of the piezoelectriclayers 11 connected thereto cannot be sufficiently reduced. On the otherhand, when the void ratio is larger than 99%, the strengths of thehigh-resistance metal layers 12 m are lowered, and there is thelikelihood that the high-resistance metal layers 12 m will be broken.

As described above, the porosity (void ratio) is a measured value of across section obtained by cutting the multilayer piezoelectric elementby a plane parallel to the stacking direction, or a plane perpendicularto the stacking direction. Specifically, the measured value is obtainedby measuring the sectional areas of voids existing in the cut surface ina single high-resistance metal layer, and dividing the obtainedsectional areas by the total of the sectional areas of thehigh-resistance metal layers 12 m, and then multiplying the result by100. Although no special limitation is imposed on the diameters of thevoids, it is preferably 3 to 100 μm, and more preferably 5 to 70 μm.

It is preferable that the high-resistance metal layers 12 m contain ahigh-resistance component having a high electrical resistance than thedifferent metal layers 12 n, and the content of the high-resistancecomponent be higher than the content of a high-resistance component inthe different metal layers 12 n. A large content of the high-resistancecomponent in the high-resistance metal layers 12 m enables formation ofthe metal layers having a high electrical resistance even if the amountof voids is substantially reduced. Even with the arrangement of aplurality of the high-resistance metal layers 12 m thus formed, thedisplacement variations can be further reduced. Although no speciallimitation is imposed on the diameter of the high-resistance component,it is preferably 0.1 to 100 μm, and more preferably 0.1 to 50 μm.

The content of the high-resistance component is preferably 40% to 99%,and more preferably 50 to 90%. The content of the high-resistancecomponent can be obtained by taking a SEM photograph of a surfaceparallel to the high-resistance metal layer 12 m, measuring the area ofthe high-resistance component in the surface, dividing the obtained areaby the entire area, and then multiplying the result by 100. Examples ofthe high-resistance component are lead zirconate titanate (PZT), leadtitanate, alumina, titania, silicon nitride and silica.

The thickness of the high-resistance metal layer 12 m is preferablysmaller than the thickness of the different metal layer 12 n. The reasonfor this is that the high-resistance metal layer 12 m having a smallerthickness than the different metal layer 12 n facilitates itsdeformation than the different metal layer 12 n, enabling a reduction inthe resistance generated in the piezoelectric layer 11 adjacent to thehigh-resistance metal layer. This permits an improvement of durability.Additionally, when the high-resistance metal layer has a smallerthickness than the different metal layer, the metal layers are easy todeform and absorb stress. This makes the metal layers difficult toflake, permitting an improvement of durability.

The metal layer thickness in the present embodiment is measured on asurface obtained by cutting the multilayer piezoelectric element in thestacking direction. Firstly, arbitrary five locations in the differentmetal layer are selected, and the thickness of each location is measuredby sandwiching each location by arbitrary two parallel lines.Specifically, one of the two parallel lines is set on the boundarybetween the metal layer and the piezoelectric layer, and the other lineis shifted to another boundary. Then, the distance between the twoparallel lines is measured. A similar measurement is made for thehigh-resistance metal layer 12 m to determine its thickness. Although nolimitation is imposed on the thickness of the high-resistance metallayer 12 m, it is preferably 30 to 0.1 μm, and more preferably 20 to 1μm. The thickness of the different metal layer 12 n is preferably 103%and above, and more preferably 110% and above to that of thehigh-resistance metal layer.

Preferably, the ratio of the electrical resistance of thehigh-resistance metal layer 12 m to the piezoelectric layer 11 is 1/10(namely 0.1) to 1000 times. Within this range, the displacement of thepiezoelectric layer 11 connected to the high-resistance layer 12 m canbe controlled suitably. When the ratio of the electrical resistance ofthe high-resistance metal layer 12 m to the piezoelectric layer 11 issmaller than 1/10, the displacement of the piezoelectric layer 11connected to the high-resistance metal layer 12 m makes no differencefrom the displacement of other piezoelectric layer 11. There is thelikelihood that sufficient stress dispersing effect cannot be obtained.On the other hand, when the ratio of the electrical resistance of thehigh-resistance metal layer 12 m to the piezoelectric layer 11 is over1000 times, the displacement of the piezoelectric layer 11 connected tothe high-resistance metal layer 12 m becomes too small, thereby beingsusceptible to stress concentration. For the same reason, the ratio ofthe electrical resistance of the high-resistance metal layer 12 m to thepiezoelectric layer 11 is preferably 1 to 1000 times.

The electrical resistance (C) in the present embodiment can be measuredas follows. That is, using a pico-ampere meter (for example, 4140B,manufactured by Hewlett-Packard Company), measurements are made in eachlayer by applying probes to both ends of the high-resistance metal layer12 m or the both ends of the piezoelectric layer 11, respectively. Asused here, “the both ends of the high-resistance metal layer 12 m” meansthe ends of the high-resistance metal layer 12 m exposed to the twoopposed side surfaces of the stacked body 13, respectively. If the endsof the high-resistance metal layer 12 m are not exposed to the sidesurfaces of the stacked body 13, polishing may be performed with a knownpolishing device or the like until the ends of the high-resistance metallayer 12 m are exposed. Thereafter, the electrical resistance ismeasured by applying the probes of the pico-ampere meter to the bothends of the high-resistance metal layer 12 m, respectively. When makingmeasurements of the electrical resistance, a suitable temperature is 25°C.

Preferably, the electrical resistance of the high-resistance metal layer12 m is 1000 times and above of the electrical resistance of thedifferent metal layer 12 n. This ensures that the displacement of thepiezoelectric layer 11 connected to the high-resistance metal layer 12 mis smaller than that of the piezoelectric layer 11 connected to thedifferent metal layer 12 n. Hence, the multilayer piezoelectric elementcan be divided by the high-resistance metal layers 12 m, so that thestress can be dispersed, permitting an improvement of durability.

Next, a description will be given of a method of manufacturing themultilayer piezoelectric element according to the twelfth preferredembodiment.

Firstly, a plurality of ceramic green sheets serving as thepiezoelectric layers 11 are manufactured. Subsequently, a conductivepaste is prepared by containing an organic matter (for example, acrylbeads), which are bindingly fixed during drying, and volatized duringsintering, in metal powder composing the high-resistance metal layers 12m, such as silver-palladium, and by adding while mixing binder andplasticizer. The conductive paste is then printed in a thickness of 1 to40 μm on the upper surfaces of a part of the above green sheets byscreen printing or the like.

Here, changing the ratio of the acryl beads and the metal powder canchange the void ratio of the high-resistance metal layers. That is, ahigh ratio of the acryl beads increases the void ratio, and a low ratioof the acryl beads decreases the void ratio. The diameter of the voidscan be adjusted by changing the diameter of the beads. Alternatively, anacryl beads paste may be prepared by adding while mixing binder andplasticizer in an organic matter such as acryl beads. Separately, aconductive paste is prepared by adding while mixing binder andplasticizer in metal powder constituting the high-resistance metallayers 12 m such as silver-palladium. The obtained acryl beads paste andthe conductive paste are then stackingly printed on the upper surfacesof a part of the above green sheets by screen printing or the like. Thispermits printing with more excellent mass production.

As the above organic matter, there are the same organic matters asexemplified in the method of manufacturing the multilayer piezoelectricelements according to the first to fourth preferred embodiments. Itbecomes easy to control the void ratio of the high-resistance metallayers 12 m by heating the above metal layers composed ofsilver-palladium so as to oxide their surfaces in advance.Alternatively, a high-resistance component, such as PZT, lead titanateor alumina, may be added to the metal layers of silver-palladium asneeded.

In the rest of the green sheets except for the one which formhigh-resistance metal layers 12 m, the conductive paste are printed byscreen printing or the like so that the different metal layers 12 n areformed. An organic matter such as acryl beads and high resistancecomponent can be added to the conductive paste as needed.

Subsequently, a plurality of the green sheets with the conductive pasteprinted thereon are stacked one upon another, thereby obtaining astacked matter. The stacked matter with a heavy stone mounted thereon isdebindered at a predetermined temperature. Thereafter, this is sinteredat 900 to 1200° C. without mounting any heavy stone thereon so thatvoids can be formed in the high-resistance metal layers 12A, therebyobtaining the stacked body 13. The inactive layers 14 may be formed inthe same manner as in the foregoing first to eleventh preferredembodiments.

Next, external electrodes 15 are formed in the same manner as in thefirst to eleventh preferred embodiments. Then, silicone rubber is filledinto the groove of the stacked body 13, and the silicone rubber iscoated on the side surfaces of the stacked body 13 in the same manner asin the first to eleventh preferred embodiments. Thereafter, the siliconerubber, which has been filled into the groove and coated on the sidesurfaces of the stacked body 13, is then cured, thereby completing themultilayer piezoelectric element of the present embodiment.

Finally, the polarization processing of the stacked body 13 is performedby connecting lead wires to the external electrodes 15, respectively,and then applying through the lead wires a dc voltage of 0.1 to 3 kV/mmto the pair of the external electrodes 15, respectively. This results ina piezoelectric actuator using the multilayer piezoelectric element ofthe present invention. Further, by connecting the lead wires to anexternal voltage supply part, and applying a voltage to the metal layers12 through the lead wires and the external electrodes 15, the respectivepiezoelectric layers 11 cause a large displacement by the reversepiezoelectric effect, thereby functioning as, for example, an automobilefuel injection valve for performing fuel injection and supply to anengine.

The configuration is otherwise similar to those described in the firstto eleventh preferred embodiments, and therefore the description thereofis omitted.

(Thirteenth Preferred Embodiment)

A thirteenth preferred embodiment related to a multilayer piezoelectricelement of the present invention will be described below. The multilayerpiezoelectric element according to the present embodiment can take thefollowing configurations.

(1) A multilayer piezoelectric element having a stacked body in which aplurality of piezoelectric layers and a plurality of metal layers arestacked alternately, in which at least one of a plurality of the metallayers is composed of a plurality of metal parts disposed between thepiezoelectric layers.

(2) The multilayer piezoelectric element as set forth in the above (1),in which a pair of external electrodes to which a plurality of the metallayers are connected are formed on side surfaces of the stacked body,respectively.

(3) The multilayer piezoelectric element as set forth in the above (1)or (2), in which a part of a plurality of the metal parts is oppositelydisposed piezoelectric layers adjacent to both ends in a thickness(stacking) direction of the metal parts, respectively, and the rest ofthe metal parts are connected through only one end thereof in thethickness direction of the metal parts to the piezoelectric layers,respectively.

(4) The multilayer piezoelectric element as set forth in any one of theabove (1) to (3), in which there are a plurality of metal layerscomposed of the metal parts.

(5) The multilayer piezoelectric element as set forth in the above (4),in which a plurality of metal layers composed of the metal parts arearranged respectively with a plurality of piezoelectric layers inbetween.

(6) The multilayer piezoelectric element as set forth in the above (4)or (5), in which a plurality of metal layers composed of the metal partsare regularly arranged.

(7) The multilayer piezoelectric element as set forth in any one of theabove (1) to (6), in which the metal parts gradually decrease orincrease in width with decreasing distance to the piezoelectric layersadjacent to the metal parts, respectively.

(8) The multilayer piezoelectric element as set forth in any one of theabove (1) to (7), in which the metal parts are composed of silver,palladium, or an alloy of these.

(9) The multilayer piezoelectric element as set forth in any one of theabove (1) to (8), in which voids exist between the metal parts adjacentto each other.

In accordance with the present embodiment, at least one of a pluralityof the metal layers is composed of a plurality of metal parts disposedbetween the piezoelectric layers. This enables the metal layers composedof the metal parts to absorb stress generated by displacements generatedwhen the piezoelectric layers are displaced. Further, the presence ofthe metal layer composed of the metal parts increases the degree offreedom of the piezoelectric layers around the metal layer, permitting alarge displacement of these piezoelectric layers. This relaxessuppression of the element deformation due to stress concentration,thereby increasing the displacement of the entire element. This alsosuppresses concentration of the stress due to the element deformation.Consequently, a large displacement is attainable, and resonancephenomena can be suppressed. Even in a long-term continuous drivingunder high voltage and high pressure, displacement variations can besuppressed, thereby achieving the multilayer piezoelectric elementhaving excellent durability.

When a part of a plurality of the metal parts is oppositely disposedpiezoelectric layers adjacent to both ends in a thickness direction ofthe metal parts, respectively, and the rest of the metal parts areconnected through only one end thereof in the thickness direction of themetal parts to the piezoelectric layers, respectively, it is capable offurther increasing the effect of relaxing the stress generated in thethickness direction when the piezoelectric layers are displaced. Whenthe metal parts gradually decrease or increase in width with decreasingdistance to the piezoelectric layers adjacent to the metal parts,respectively, it is capable of suppressing the contours of the metalparts from having an acute angle, thereby suppressing stressconcentration along with the element deformation generated at the acuteangle portions. It is therefore capable of providing the multilayerpiezoelectric element having excellent durability and high reliabilityeven in a long-term continuous driving under high voltage and highpressure. That is, an injector contains the multilayer piezoelectricelement as set forth in any of the above (1) to (9), in a containerhaving an injection hole. This injector has a container having aninjection hole, and the multilayer piezoelectric element as set forth inany one of the above (1) to (9). The injector is configured so that aliquid filled in the container is discharged from the injection hole bythe driving of the multilayer piezoelectric element.

A multilayer piezoelectric element according to the present inventionwill next be described in detail with reference to the accompanyingdrawings. FIG. 19( a) is a perspective view showing a multilayerpiezoelectric element according to the present embodiment. FIG. 19( b)is a partial perspective view showing a stacked state of a piezoelectriclayer and a metal layer in FIG. 19( a). In FIG. 19, similar orequivalent parts to the configurations of FIGS. 1 to 18 as describedabove have similar reference numbers, and the description thereof isomitted.

As shown in FIGS. 19( a) and 19(b), the multilayer piezoelectric elementof the present embodiment has a stacked body 13 configured byalternately stacking a plurality of piezoelectric layers 11 and aplurality of metal layers 12 (12 o, 12 p). A pair of external electrodes15 are disposed on the opposed side surfaces of the stacked body 13 (oneof the external electrodes is not shown). The respective metal layers 12are not formed entirely over the main surfaces of the piezoelectriclayers 11, thereby forming the so-called partial electrode structure.The metal layers 12 in the partial electrode structure are arranged soas to expose by every other layer to the opposed side surfaces of thestacked body 13. Thus, the metal layers 12 are connected by every otherlayer to the pair of external electrodes 15, respectively.

Here, the multilayer piezoelectric element of the present embodiment is,as shown in FIGS. 19( a) and 19(b), at least one of a plurality of metallayers 12 is a metal layer 12 p composed of a plurality of metal parts12 q disposed between the piezoelectric layers 11. The presence of atleast one layer of the metal layer 12 p enables an increase in thedisplacement of the entire multilayer piezoelectric element, and also animprovement of the durability of the multilayer piezoelectric element.That is, as in a conventional multilayer piezoelectric element, if allof metal layers are substantially equalized in order to uniformly applyan electric field to all of piezoelectric bodies, the element itselfcontinuously causes dimensional changes during driving. Therefore, allof the piezoelectric bodies are closely driven through the metal layers,so that the multilayer piezoelectric element is drivingly deformed as awhole. As a result, the stress due to the element deformation is liableto be concentrated at the outer periphery of the center of the elementwhich expands at the time of compression and necks at the time ofspreading. Particularly, there is a tendency that the stressconcentrates at the boundary between an inactive layer causingpiezoelectric displacement and an active layer causing no piezoelectricdisplacement. In addition, there is the following problem. That is, insome cases, resonance phenomena that the displacement behaviors of therespective piezoelectric layers match with each other is generated whichmay cause beat sound, and harmonic signals of integral multiples ofdriving frequency is generated which may cause noise composition.

On the other hand, in the multilayer piezoelectric element of thepresent embodiment, at least one layer of the metal layers 12 is themetal layer 12 p. This decreases the displacement of the piezoelectriclayer 11 around the metal layer 12 p, and increases the displacement ofthe piezoelectric layer 11 around the metal layer 12 o, therebydispersing in the element locations having a large displacement andlocation having a small displacement. The arrangement of these metallayers in the element enables dispersion of the stress exerted on theelement. As a result, the suppression of the element deformation due tostress concentration can be relaxed, thereby increasing the displacementof the entire element. It is also capable of suppressing concentrationof the stress due to the element deformation, thereby exhibitingexcellent durability even in a long-term continuous driving under highvoltage and high pressure.

A plurality of metal parts 12 q constituting the metal layer 12 p arepreferably arranged substantially uniformly between the piezoelectriclayers. When the metal parts 12 q are arranged substantially uniformlybetween the piezoelectric layers, the stress along with the elementdeformation cannot be concentrated at a part, and the metal layer 12 pfunctions as a stress relaxing layer of the piezoelectric layersthroughout the cross section of the element.

In the present embodiment, a plurality of the metal layer 12 p exist inthe stacked body 13. The respective metal layers 12 p are arrangedinterposing in between a plurality of the piezoelectric layers 11 and aplurality of the metal layers 12 o, and are arranged regularly in thethickness direction of the stacked body 13. The portions of a pluralityof the piezoelectric layers 11 which are drivingly deformed correspondto the layer sandwiched between the metal layers 12 o. Therefore, byforming the metal layer 12 p at the locations of the metal layers 12 inwhich a plurality of the piezoelectric layers 11 are interposed, it iscapable of securing the element displacement to a certain degree, andalso suppressing the occurrence of resonance phenomena to be generatedwhen the displacement behaviors of the respective piezoelectric layersmatch with each other, thereby preventing beat sound generation. It isalso capable of preventing harmonic signal generation, therebysuppressing the noise of control signals. Additionally, the magnitude ofthe displacement of the piezoelectric layers 11 can be controlled bychanging the thicknesses of the metal layers 12. This eliminates theneed to change the thicknesses of the piezoelectric layers, permittingthe structure that is effective for mass production.

In the present embodiment, it is desirable that a part of a plurality ofthe metal parts 12 q constituting the metal layer 12 p be connected tothe piezoelectric layers adjacent to both ends in the thicknessdirection of the metal parts 12 q, respectively, and the rest of themetal parts 12 q constituting the metal layer 12 p be connected throughonly one end thereof in the thickness direction of the metal parts 12 qto the piezoelectric layers 11, respectively. One of the functionsrequired for the metal layer 12 p is to increase the displacement duringthe time the multilayer piezoelectric element is driven. This requiresthat each of the metal parts 12 q constituting the metal layer 12 p isconnected through both ends or one end thereof in the thicknessdirection thereof to the adjacent and oppositely disposed piezoelectriclayers 11. When both ends in the thickness direction of the metal parts12 q constituting the metal layer 12 p are unconnected to the adjacentand oppositely disposed piezoelectric layers 11, a sufficient springfunction of connecting these piezoelectric layers 11 cannot be imparted,and in some cases, failing to obtain sufficiently the effect ofincreasing the displacement during the time the multilayer piezoelectricelement is driven.

It is also desirable that in a region proximate to the adjacentpiezoelectric layers 11, a plurality of the metal parts 12 qconstituting the metal layer 12 p gradually decrease or increase inwidth with decreasing distance to these piezoelectric layers,respectively. Here, the other function required for the metal layer 12 pis to relax the stress generated when the multilayer piezoelectricelement is driven and displaced. In order to attain this function, it isrequired to relax the stress generated on the interface between thepiezoelectric layer 11 and the metal layer 12, without causing thestress to be concentrated at a point. In the present embodiment, for thepurpose of further improving this stress relaxing function, particularlyin a region proximate to the adjacent piezoelectric layers 11, aplurality of the metal parts 12 q constituting the metal layer 12 pgradually decrease or increase in width with decreasing distance tothese piezoelectric layers, respectively, so as to suppress the stressconcentrating at a point. Consequently, the piezoelectric layers 11connected to the metal layer 12 p are free from stress concentration,permitting a large displacement. It is therefore capable of retainingthe driving displacement of the element to be retained, and avoiding theelement stress from concentrating at a point. This provides apiezoelectric actuator having a large displacement, excellent durabilityand high durability.

It is desirable that voids exist between a plurality of the metal parts12 q adjacent to each other in the metal layer 12 p. The reason for thisis as follows. In the presence of an insulating material other than themetal component of the metal layer 12 p, when the element is driven, aportion where no voltage can be applied to the piezoelectric layer 11may be generated, and in some cease, piezoelectric displacement cannotbe increased sufficiently. In addition, the stress during driving isliable to be concentrated.

On the other hand, in the presence of voids between a plurality of themetal parts 12 q constituting the metal layer 12 p, when stress isexerted on metal portions, the void portions can cause the metal parts12 q to be deformed and dispersedly relax the stress. Additionally, whenthe piezoelectric layers 11 connected to the metal layer 12 p causepiezoelectric displacement, the presence of the void portions enablepartial cramping of the piezoelectric layers 11, so that the forceconstraining the piezoelectric layers 11 can be reduced than whencramping by the entire surface. As a result, the piezoelectric layers 11are easy to deform, permitting a large displacement. This achieves themultilayer piezoelectric element exhibiting a larger elementdisplacement and high durability.

Also in the present embodiment, the metal composing the metal layer 12 pis preferably silver, palladium, or a compound of these. Since thesemetals have high thermal distance, it becomes possible to performsimultaneous sintering of the piezoelectric layers 11 having a highsintering temperature, and the metal layers 12. This permits themanufacture in which the sintering temperature of the externalelectrodes is set to a lower temperature than the sintering temperatureof the piezoelectric layers 11, thereby suppressing severe mutualdispersion between the piezoelectric layers 11 and the externalelectrodes 15.

Next, a description will be given of a method of manufacturing themultilayer piezoelectric element according to the thirteenth preferredembodiment.

Firstly, ceramic green sheets serving as the piezoelectric layers 11 aremanufactured in the same manner as in the first to twelfth preferredembodiments. Subsequently, a conductive paste is prepared by addingwhile mixing binder and plasticizer in metal powder constituting themetal layers 12 of silver-palladium or the like. The conductive paste isthen printed in a thickness of 1 to 40 μm on the upper surfaces of eachof the green sheets by screen printing or the like.

Here, the thicknesses of the metal layers 12 and the voids in the metallayers can be changed by changing the ratio of the binder and theplasticizer to the metal powder, or changing the degree of a screenmesh, or changing the thickness of a resist for forming a screenpattern.

Subsequently, a plurality of the green sheets with the conductive pasteprinted thereon are stacked one upon another. With a heavy stone mountedthereon, this stacked matter is debindered at a predeterminedtemperature. Thereafter, this is sintered at 900 to 1200° C. withoutmounting any heavy stone thereon so that the metal layers have differentthicknesses, thereby obtaining the stacked body 13. The inactive layers14 may be formed in the same manner as in the foregoing first to twelfthpreferred embodiments.

Thereafter, the metal layer 12 whose end is exposed to the side surfaceof the multilayer piezoelectric element, and the metal layer 12 (12 o or12 p), whose end is not exposed thereto, are alternately formed. Then, agroove is formed in a piezoelectric portion between the metal layer 12whose end is not exposed, and the external electrodes 15. An insulatorof resin or rubber, having a lower Young's modulus than thepiezoelectric layers 11, is formed in the groove. Here, the groove isformed of the side surface of the stacked body 13 by using an internaldicing device or the like.

Next, external electrodes 15 are formed in the same manner as in thefirst to twelfth preferred embodiments. Silicone rubber is filled intothe groove of the stacked body 13, and silicone rubber is coated on theside surfaces of the stacked body 13 in the same manner as in the firstto twelfth preferred embodiments. The silicone rubber, which is filledinto the groove and also coated on the side surfaces of the stacked body13, is then cured, thereby obtaining the multilayer piezoelectricelement of the present embodiment.

Finally, the polarization processing of the stacked body 13 is performedby connecting lead wires to the external electrodes 15, respectively,and then applying through the lead wires a dc voltage of 0.1 to 3 kV/mmto the pair of the external electrodes 15, respectively. This results ina piezoelectric actuator using the multilayer piezoelectric element ofthe present embodiment.

The configuration is otherwise similar to those described in the firstto twelfth preferred embodiments, and therefore the description thereofis omitted.

<Injector>

A preferred embodiment of an injector provided with the multilayerpiezoelectric element of the present invention as described above willbe described in detail below with reference to the drawing. FIG. 20 is aschematic cross section showing an injector according to the presentembodiment. As shown in FIG. 20, in the injector of the presentembodiment, a piezoelectric actuator 43 having the multilayerpiezoelectric element of the present invention represented by theforegoing preferred embodiments is contained in the inside of acontainer 31 having at one end thereof an injection hole 33.

Specifically, a needle valve 35 capable of opening and closing theinjection hole 33 is disposed in the container 31. A fuel passage 37 isdisposed in the injection hole 33 so as to permit communication inresponse to movement of the needle valve 35. The fuel passage 37 isconnected to an external fuel supply source, and fuel is normallysupplied to the fuel passage at a constant high pressure. Therefore, itis configured so that when the needle valve 35 opens the injection 33,the fuel being supplied at a constant high pressure to the fuel passage37 is jetted into a fuel room of an internal combustion engine (notshown).

The upper end of the needle valve 35 has a large internal diameter, andaccommodates a cylinder 39 and a slidable piston 41 which are formed inthe container 31.

In the above injector, when a piezoelectric actuator 43 spreads uponapplication of a voltage, the piston 41 is pressed, and the needle valve35 closes the injection hole 33, thereby stopping the fuel supply. It isalso configured so that when the voltage application is stopped, thepiezoelectric actuator 43 contracts, and a coned disc spring 45 pushesback the piston 41, and the injection hole 33 is communicated with thefuel passage 37, thereby performing fuel jetting.

While the embodiment of the present invention has been described above,the present invention is not limited to the above preferred embodiment.For example, although the above embodiment has described the case wherethe multilayer piezoelectric element is applied to the injector, thepresent invention is not limited to this case, and applicable to, forexample, drive elements mounted on precision positioners, shockprevention devices and the like in fuel injectors of automobile engines,liquid injectors such as ink jets, optical devices and the like, oralternatively, to sensor elements mounted on combustion pressuresensors, knock sensors, acceleration sensors, load sensors, ultrasonicsensors, pressure sensitive sensors, yaw rate sensors and the like, aswell as circuit elements mounted on piezoelectric gyroscopes,piezoelectric switches, piezoelectric transformers, piezoelectricbreakers and the like. Besides these, it is possible to practice as longas being elements using piezoelectric characteristics.

Hereinafter, the present invention will be described in further detailby illustrating examples, without limiting the present invention to thefollowing examples.

Example I-a

<Manufacturing of Piezoelectric Actuators>

Piezoelectric actuators composed of a multilayer piezoelectric elementwere manufactured as follows.

Firstly, slurry was prepared which a mixture of a calcined powder ofcomposed mainly of lead zirconate titanate (PbZrO₃—PbTiO₃) having a meanparticle size of 0.4 μm, binder and plasticizer. Then, a plurality ofceramic green sheets constituting piezoelectric layers 11 having athickness of 150 μm were prepared by doctor blade method. Subsequently,main metal layers 12 a, low-filled metal layers 12 b and high-filledmetal layers 12 c were printed on one surface of each of the ceramicgreen sheets by screen printing, respectively.

Specifically, the main metal layers 12 a, the low-filled metal layers 12b and the high-filled metal layers 12 c were printed in the followingmanner.

Main metal layers 12 a: To silver-palladium alloy (95% by mass of silverand 5% by mass of palladium), 10 parts by mass of acryl beads having amean particle diameter of 0.2 μm was added with respect to 100 parts bymass of silver-palladium alloy, and binder was also added to obtain aconductive paste. The conductive paste was printed in a thickness of 3μm on one surface of each of the sheets.

Low filled metal layers 12 b: A conductive paste obtained by addingbinder to silver-palladium alloy (95% by mass of silver and 5% by massof palladium) was printed in a thickness of 1 μm on one surface of eachof the sheets. An acryl beads paste obtained by adding binder to acrylbeads having a mean particle diameter of 1 μm was stackingly printed ina thickness of 10 μm on the conductive paste. The acryl beads wereblended so as to constitute 5 parts by mass with respect to 100 parts bymass of silver-palladium alloy.

High filled metal layers 12 c: A conductive paste obtained by addingbinder to silver-palladium alloy (95% by mass of silver and 5% by massof palladium) was printed in a thickness of 3 μm on one surface of eachof the sheets.

There were prepared 300 sheets on which the respective metal layers werethus printed. Separately, green sheets constituting inactive layer 14were prepared. These two kinds of sheets were stacked so that 30 piecesof the inactive layers, 300 pieces of stacked bodies and 30 pieces ofthe inactive layers were stacked in this order and from bottom to top,thereby obtaining a stacked matter.

The stacking was made in the combinations shown in Table 1. The detailsin Table 1 are as follows.

“Rate of layer numbers of the main metal layers 12 a” means that therate (%) of the layer numbers of the main metal layers 12 a to the totalmetal layer numbers.

“Opposite arrangement of the low-filled metal layers 12 b and thehigh-filled metal layers 12 c” means that whether or not the low-filledmetal layers 12 b and the high-filled metal layers 12 c are oppositelyarranged with at least one layer of the piezoelectric layer 11 inbetween.

“The high-filled metal layers 12 c as being metal layers disposed to theboth sides of the low-filled metal layer 12 b” means that whether or notmetal layers adjacent and opposed to the low-filled metal layer 12 b inthe stacking direction are the high-filled metal layers 12 c.

“The main metal layers 12 a stacked in descending order of the metalfilling rate from bottom to top” means that whether or not thelow-filled metal layer 12 b, the high-filled metal layer 12 c and themain metal layer 12 a are arranged in the order named and in thestacking direction, with the piezoelectric layer 11 in between, and alsothe main metal layers 12 a are stacked in descending order of the metalfilling rate from the high-filled metal layer 12 c.

The respective numeral values in the column of “the presence and absenceof low-filled metal layer 12 b” in Table 1 indicates to which layerorder in the stacking direction the low-filled metal layer 12 bcorresponds. Similarly, the respective numeral values in the column of“the presence and absence of high-filled metal layer 12 c” in Table 1indicates to which layer in the stacking direction the high-filled metallayer 12 c corresponds.

The stacked matter was pressed, debindered and sintered. In thesintering process, the stacked body was retained at 800° C. for twohours, and then sintered at 1000° C. for two hours, thereby obtaining astacked body 13. The metal filling rate of the metal layers 12 a to 12 cin the stacked body 13 were measured. The results are as follows.

The metal filling rate X1 in the main metal layer 12 a was 70%.

The metal filling rate Y1 in the low-filled metal layer 12 b was 45%.

The metal filling rate Z1 in the high-filled metal layer 12 c was 85%.

Next, to a mixture of flake-shaped silver powder having a mean particlesize of 2 μm, and amorphous glass powder composed mainly of silicon,having a mean particle size of 2 μm and a softening point of 640° C., 8parts by mass of binder was added with respect to 100 parts by mass ofthe mixture of the silver powder and the glass powder, and this wassufficiently mixed together to obtain a silver glass conductive paste.Subsequently, the silver glass conductive paste was printed on a moldreleasing film by screen printing. This was dried and then separatedfrom the mold releasing film, thereby obtaining a silver glassconductive paste sheet. The silver glass conductive paste sheet was thentransferred to and stacked on the surfaces on which external electrodes15 should be formed. This was then baked at 700° C. for 30 minutes,thereby forming the external electrodes 15, resulting in a multilayerpiezoelectric element. The mean particle size of the flake-shaped powderwas measured as follows. That is, a photograph of this powder was takenby using a scanning electron microscope (SEM). A line was drawn on thephotograph, and 50 pieces of lengths over which particles and the lineare crossed were measured and averaged. The result was employed as amean particle size.

Then, the polarization processing was performed by connecting lead wiresto the external electrodes 15 of the obtained multilayer piezoelectricelement, respectively, and by applying through the lead wires a dcelectric field of 3 kV/mm for 15 minutes to the positive electrode andthe negative electrode of the external electrodes 15, respectively.Thus, each piezoelectric actuator using the multilayer piezoelectricelement as shown in FIG. 1 was manufactured (Sample Nos. I-1 to I-9 inTable 1). By applying a dc current of 170V to the obtained multilayerpiezoelectric element, every piezoelectric actuator had a displacementin the stacking direction.

<Evaluations>

A continuous driving test was conducted for each of the obtainedpiezoelectric actuators. Its evaluation method was as follows. Theevaluation results are shown in Table 1.

(Evaluation Method of the Continuous Driving Test)

In the test, each piezoelectric actuator was continuously driven up to1×10⁹ times by applying an alternating voltage of 0 to +170V at afrequency of 150 Hz at room temperature. More specifically, the test wasconducted using 100 pieces per sample. The displacements were measuredwith an optical non-contact micro-displacement meter. The displacementin the initial state means the displacement when driven a time. Using ametal microscope and an SEM or the like, the stacked portions after thecontinuous driving were observed to confirm the presence and absence ofdelamination. Further, the presence and absence of the noise generationof harmonic components, and the presence and absence of beat soundgeneration at 1 kHz were evaluated.

TABLE 1 Existence Existence Opposite arrangement of low-filled ofhigh-filled of low-filled metal Constitution Rate of layer numbers metallayers 12b metal layers 12c layers 12b and high- Sample No. of metallayers of metal layers 12a (Position) (Position) filled metal layers 12cI-1 FIG. 2 98% Existence None None 50, 100, 150, 200, 250 I-2 FIG. 7 98%None Existence None 50, 100, 150, 200, 250 I-3 FIG. 4 96% ExistenceExistence Existence 50, 100, 200, 250 49, 99, 201, 251 I-4 FIG. 4 96%Existence Existence Existence 49, 99, 201, 251 50, 100,, 200, 250 I-5FIG. 5 95% Existence Existence Existence 50, 100, 150, 200, 250 49, 51,99, 101, 149, 151, 199, 201, 249, 251 I-6 FIG. 5 93% Existence ExistenceExistence 1, 50, 100, 150, 200, 250, 300 2, 49, 51, 99, 101, 149, 151,199, 201, 249, 251, 299 I-7 FIG. 5 92% Existence Existence Existence 2,50, 100, 150, 200, 250, 299 1, 3, 49, 51, 99, 101, 149, 151, 199, 201,249, 251, 298, 300 I-8 FIG. 5 95% Existence Existence Existence 50, 100,150, 200, 250 49, 51, 99, 101, 149, 151, 199, 201, 249, 251 I-9 FIG. 22100%  None None None Result of continuous driving test Metal layersDisplacement High-filled metal layers 12a stacked in after Noise 12c asbeing metal layers descending Displacement continuous generation ofdisposed to both sides of order of metal in initial state drivingharmonic Beat sound Sample No. low-filled metal layers 12b filling rate(μm) (μm) Delamination components generation I-1 None None 50.0 49.9None None None I-2 None None 50.0 49.9 None None None I-3 None None 55.054.9 None None None I-4 None None 55.0 54.9 None None None I-5 ExistenceNone 60.0 59.9 None None None I-6 Existence None 60.0 60.0 None NoneNone I-7 Existence None 60.0 60.0 None None None I-8 Existence Existence60.0 60.0 None None None I-9 None None 45.0 42.0 Occurred OccurredOccurred

As apparent from Table 1, in Sample No. 1-9 as a comparative example,the stress exerted on the stacking interface was concentrated at apoint, which increased load and caused delamination (inter-layerpeeling), as well as beat sound generation and noise generation. On theother hand, in Samples Nos. I-1 to I-8 as the present invention, noremarkable drop in the element displacement was confirmed even afterbeing continuously driven 1×10⁹ times, and hence these samples hadeffective displacements required for the piezoelectric actuators,respectively. This shows that the piezoelectric actuators havingexcellent durability were achieved.

Particularly, it can be seen that Samples Nos. I-3 and I-4, in which astress relaxing layer (the low-filled metal layer 12 b) and a stressconcentrating layer (the high-filled metal layer 12 c) were adjacent toeach other with the piezoelectric layer 11 in between, were capable ofincreasing the element displacement and also manufacturing themultilayer actuators exhibiting a stable element displacement. It canalso be seen that Samples Nos. I-5 to I-8, in which the stress relaxinglayers were interposed with the piezoelectric layer 11 in between, werecapable of achieving the largest element displacement, and alsomanufacturing the piezoelectric actuators which caused little change ofthe element displacement and had extremely excellent durability, therebyexhibiting a stable element displacement.

Example I-b

Piezoelectric actuators were obtained (Samples Nos. I-10 to I-15 inTable 2) by changing the compositions (Y1/X1 and Z1/X1) of the metallayers 12 in the piezoelectric actuators of Sample No. I-8 in the aboveExample I-a, into those as shown in Table 2. As a comparative example,the piezoelectric actuator of Sample No. I-9 in Example I-a was alsopresented (Sample No. I-15 in Table 2). By applying a dc voltage of 170Vto each of the obtained multilayer piezoelectric elements, everypiezoelectric actuator had a displacement in the stacking direction.Since in the piezoelectric actuator of Sample No. I-15, the metalfilling rates of all of the metal layers were set to about 70%, theseare expressed as X1=70%, Y1=70% and Z1=70%, and the filling rate ratiowas expressed as Y1/X1=1, and Z1/X1=1 in Table 2.

In the same manner as in Example I-a, a continuous driving test wasconducted for each of the obtained piezoelectric actuators (Samples Nos.I-10 to I-15 in Table 2). The results are shown in Table 2.

TABLE 2 Result of continuous driving test Displacement NoiseConstitution Displacement after generation of Sample of metal X1 Y1 Z1in initial state continuous harmonic Beat sound No. layer (%) (%) (%)Y1/X1 Z1/X1 (μm) driving (μm) Delamination components generation I-10FIG. 5 48 4.8 96 0.1 2 60.0 60.0 None None None I-11 FIG. 5 66 20 99 0.31.5 65.0 65.0 None None None I-12 FIG. 5 63 32 76 0.5 1.2 70.0 70.0 NoneNone None I-13 FIG. 5 72 36 80 0.8 1.1 70.0 70.0 None None None I-14FIG. 5 66 60 70 0.9 1.05 65.0 65.0 None None None I-15 FIG. 22 70 70 701 1 45.0 42.0 Occurred Occurred Occurred

As apparent from Table 2, in Sample No. I-15, in which Y1/X1 was largerthan 0.9, and Z1/X1 was smaller than 1.05, the stress exerted on thestacking interface was concentrated at a point, which increased load andcaused delamination (inter-layer peeling), as well as beat soundgeneration and noise generation.

On the other hand, in Samples Nos. I-10 to I-14, in which Y1/X1 was inthe range of 0.1 to 0.9, and Z1/X1 was in the range of 1.05 to 2, werecapable of achieving the largest element displacement and also achievingthe multilayer actuators which caused little change of the elementdisplacement and had extremely excellent durability, thereby exhibitinga stable element displacement. In particular, Samples Nos. I-12 andI-13, in which Y1/X1 was in the range of 0.5 to 0.8, and Z1/X1 was inthe range of 1.1 to 1.2, were capable of achieving the multilayeractuators having excellent element displacement.

Example I-c

Piezoelectric actuators were obtained (Samples Nos. I-16 to I-33 inTable 3) by changing the material compositions of the metal layers 12 inthe piezoelectric actuators of Sample No. I-8 in the above Example I-a,into those as shown in Table 3. By applying a dc voltage of 170V to eachof the obtained multilayer piezoelectric elements, every piezoelectricactuator had a displacement in the stacking direction.

In the same manner as in Example I-a, a continuous driving test wasconducted for each of the obtained piezoelectric actuators. Each rate ofchange of the displacement (%) was calculated by introducing thedisplacement of the initial state and the displacement after thecontinuous driving into the equation: [1-(Displacement after continuousdriving)/(Displacement of initial state)]×100. The results are shown inTable 3.

TABLE 3 Result of continuous Material compositions of driving test metallayers 12 Rate of change of Sample Pd Ag Cu Ni displacement No. (% bymass) (% by mass) (% by mass) (% by mass) (%) I-16 0.001 99.999 0 0 0.7I-17 0.01 99.99 0 0 0.7 I-18 0.1 99.9 0 0 0.4 I-19 0.5 99.5 0 0 0.2 I-201 99 0 0 0.2 I-21 2 98 0 0 0 I-22 4 95 1 0 0 I-23 5 95 0 0 0 I-24 8 92 00 0 I-25 9 91 0 0 0.2 I-26 9.5 90.5 0 0 0.2 I-27 10 90 0 0 0.4 I-28 1585 0 0 0.7 I-29 0 0 100 0 0.2 I-30 0 0 99.9 0.1 0 I-31 0 0 0 100 0.4I-32 20 80 0 0 0.9 I-33 30 70 0 0 0.9

As apparent from Table 3, in Sample Nos. I-32 and I-33, the content ofmetal of groups 8 to 10 in the metal composition in the metal layers 12was above 15% by mass, and the content of metal of group 11 was below85%. Consequently, the metal layers 12 had a large specific resistance,and when the multilayer piezoelectric element was continuously driven,heat was generated, and the displacement of the piezoelectric actuatorwas lowered.

On the other hand, Samples Nos. I-16 to I-28 were composed mainly of ametal composition satisfying the following relationship of: 0<M1≦15,85≦M2<100, M1+M2=100% by mass, where M1% by mass is a content of metalin groups 8 to 10 in the metal layers 12, and M2% by mass is a contentof a metal in group 11b. It was therefore capable of reducing thespecific resistance of the metal layers 12, and suppressing the heatgeneration occurred in the metal layers 12 even in the continuousdriving, thereby manufacturing the multilayer actuators with a stableelement displacement. It can also be seen that Samples Nos. 1-29 to I-31were capable of reducing the specific resistance of the metal layers 12,and suppressing the heat generation occurred in the metal layers 12 evenin the continuous driving, thereby manufacturing the multilayeractuators with a stable element displacement.

Example I-d

<Manufacturing of Piezoelectric Actuators>

Piezoelectric actuators composed of a multilayer piezoelectric elementwere manufactured as follows.

In the same manner as in Example I-a, there were prepared 30 sheets onwhich the respective metal layers were printed. Separately, green sheetsconstituting inactive layer 14 were prepared. These two kinds of sheetswere stacked so that 5 pieces of the inactive layers, 30 pieces ofstacked bodies and 5 pieces of the inactive layers were stacked in thisorder and from bottom to top, thereby obtaining a stacked matter.

The stacking was made in the combinations shown in Table 4. The detailsin Table 4 are as follows.

“Arrangement of the low-filled metal layers 12 b and the high-filledmetal layers 12 c” means that whether or not the low-filled metal layers12 b and the high-filled metal layers 12 c are oppositely arranged withat least one layer of the piezoelectric layer 11 in between.

The stacked matter was pressed, debindered and sintered. In thesintering process, the stacked body was retained at 800° C. for twohours, and then sintered at 1000° C. for two hours, thereby obtaining astacked body 13. The metal filling rate of the metal layers 12 a to 12 cin the stacked body 13 were measured. The results are as follows.

The metal filling rate X1 in the main metal layer 12 a was 70%.

The metal filling rate Y1 in the low-filled metal layer 12 b was 45%.

The metal filling rate Z1 in the high-filled metal layer 12 c was 85%.

Next, in the same manner as in Example I-a, each multilayerpiezoelectric element was obtained by forming external electrodes 15 onthe stacked body 13. Then, the polarization processing was performed byconnecting lead wires to the external electrodes 15 of the obtainedmultilayer piezoelectric element, respectively, and by applying throughthe lead wires a dc electric field of 3 kV/mm for 15 minutes to thepositive electrode and the negative electrode of the external electrodes15, respectively. Thus, each piezoelectric actuator using the multilayerpiezoelectric element as shown in FIG. 1 was manufactured (Sample Nos.I-34 to I-37 in Table 4). By applying a dc current of 170V to theobtained multilayer piezoelectric element, every piezoelectric actuatorhad a displacement in the stacking direction.

<Evaluations>

In the same manner as in Example I-a, a continuous driving test wasconducted for each of the obtained piezoelectric actuators. Theevaluation results are shown in Table 4.

TABLE 4 Result of continuous driving test Opposite arrangementDisplacement Existence of low-filled Existence of high-filled oflow-filled metal Displacement after continuous Sample metal layers 12bmetal layers 12c layers 12b and high- in initial state driving No.(Position) (Position) filled metal layers 12c (μm) (μm) DelaminationI-34 Existence None None 5.0 4.9 None 1, 30 I-35 None Existence None 5.55.4 None 1, 30 I-36 Existence Existence Existence 5.5 5.5 None 1, 30 2,29 I-37 None None None 5.0 4.2 Occurred

As apparent from Table 4, in Sample No. I-37 as a comparative example,the stress exerted on the stacking interface was concentrated at apoint, which increased load and caused delamination (inter-layerpeeling), as well as beat sound generation and noise generation. On theother hand, in Samples Nos. I-34 to I-36 as the present invention, noremarkable drop in the element displacement was confirmed even afterbeing continuously driven 1×10⁹ times, and hence these samples hadeffective displacements required for the piezoelectric actuators,respectively. This shows that the piezoelectric actuators havingexcellent durability were achieved.

Particularly, it can be seen that Sample No. I-36, in which the stressrelaxing layer (the low-filled metal layer 12 b) and the stressconcentrating layer (the high-filled metal layer 12 c) were adjacent toeach other with the piezoelectric layer 11 in between, were capable ofincreasing the element displacement and also manufacturing themultilayer actuators exhibiting a stable element displacement.

Example II-a

<Manufacturing of Piezoelectric Actuators>

Piezoelectric actuators composed of a multilayer were manufactured asfollows.

Firstly, in the same manner as in Example I-a, a plurality of ceramicgreen sheets composed of a piezoelectric layer 11 having a thickness of150 μm were prepared. Subsequently, using a conductive paste obtained byadding binder to silver-palladium alloy (95% by mass of silver and 5% bymass of palladium), a main metal layer 12 d, a thin metal layer 12 e anda thick metal layer 12 f were printed on one surface of each of theabove green sheets by screen printing, respectively.

Specifically, the main metal layer 12 d, the thin metal layer 12 e andthe thick metal layer 12 f were printed as follows.

The main metal layer 12 d was printed in a thickness of 5 μm by aprocess using a resist thickness of 10 μm.

The thin metal layer 12 e was printed in a thickness of 1 μm by aprocess using a resist thickness of 2 μm.

The main metal layer 12 f was printed in a thickness of 10 μm by aprocess using a resist thickness of 20 μm.

There were prepared 300 sheets on which the respective metal layers werethus printed. Separately, green sheets constituting inactive layer 14were prepared. These two kinds of sheets were stacked so that 30 piecesof the inactive layers, 300 pieces of stacked bodies and 30 pieces ofthe inactive layers were stacked in this order and from bottom to top,thereby obtaining a stacked matter.

The stacking was made in the combinations shown in Table 5. The detailsin Table 5 are as follows.

“Rate of layer numbers of the metal layers 12 d” means that the rate (%)of the layer numbers of the main metal layers 12 d to the total metallayer numbers.

“Opposite arrangement of the thin metal layers 12 e and the thick metallayers 12 f” means that whether or not the thin metal layers 12 e andthe thick metal layers 12 f are oppositely arranged with at least onelayer of the piezoelectric layer 11 in between.

“The thick metal layers 12 f as being metal layers disposed to the bothsides of the thin metal layer 12 e” means that whether or not metallayers adjacent and opposed to the thin metal layer 12 e in the stackingdirection are the thick metal layers 12 f.

“The metal layers 12 d stacked in descending order of thickness frombottom to top” means that whether or not the thin metal layer 12 e, thethick metal layer 12 f and the main metal layer 12 d are arranged in theorder named and in the stacking direction, with the piezoelectric layer11 in between, and also the main metal layers 12 d are stacked indescending order of thickness from bottom to top.

The stacked matter was pressed, debindered and sintered. In thesintering process, the stacked body was retained at 800° C. for twohours, and then sintered at 1000° C. for two hours, thereby obtaining astacked body 13. The thicknesses of the metal layers 12 d to 12 f in thestacked body 13 were measured. The results are as follows.

The thickness X2 of the main metal layer 12 d was 5 μm.

The thickness Y2 of the thin metal layer 12 e was 2 μm.

The thickness Z2 of the thick metal layer 12 f was 7 μm.

Next, in the same manner as in Example I-a, each multilayerpiezoelectric element was obtained by forming external electrodes 15 onthe stacked body 13. Then, the polarization processing was performed byconnecting lead wires to the external electrodes 15 of the obtainedmultilayer piezoelectric element, respectively, and by applying throughthe lead wires a dc electric field of 3 kV/mm for 15 minutes to thepositive electrode and the negative electrode of the external electrodes15, respectively. Thus, each piezoelectric actuator using the multilayerpiezoelectric element as shown in FIG. 1 was manufactured (Sample Nos.II-1 to II-9 in Table 5). By applying a dc current of 170V to theobtained multilayer piezoelectric element, every piezoelectric actuatorhad a displacement in the stacking direction.

<Evaluations>

In the same manner as in Example I-a, a continuous driving test wasconducted for each of the obtained piezoelectric actuators. Theevaluation results are shown in Table 5.

TABLE 5 Rate of Opposite layer arrangement of Constitution numbers ofExistence of thin metal Existence of thick metal thin metal layersSample of metal metal layers layers 12e layers 12f 12e and thick No.layers 12a (Position) (Position) metal layers 12f II-1 FIG. 8 98%Existence None None 50, 100, 150, 200, 250 II-2 FIG. 13 98% NoneExistence None 50, 100, 150, 200, 250 II-3 FIG. 10 96% ExistenceExistence Existence 50, 100, 200, 250 49, 99, 201, 251 II-4 FIG. 10 96%Existence Existence Existence 49, 99, 201, 251 50, 100, 200, 250 II-5FIG. 11 95% Existence Existence Existence 50, 100, 150, 200, 250 49, 51,99, 101, 149, 151, 199, 201, 249, 251 II-6 FIG. 11 93% ExistenceExistence Existence 1, 50, 100, 150, 200, 250, 300 2, 49, 51, 99, 101,149, 151, 199, 201, 249, 251, 299 II-7 FIG. 11 92% Existence ExistenceExistence 2, 50, 100, 150, 200, 250, 299 1, 3, 49, 51, 99, 101, 149,151, 199, 201, 249, 251, 298, 300 II-8 FIG. 11 95% Existence ExistenceExistence 50, 100, 150, 200, 250 49, 51, 99, 101, 149, 151, 199, 201,249, 251 II-9 FIG. 23 100% None None None Thick metal layers 12f asResult of continuous driving test being metal Metal layers Displacementlayers disposed 12d stacked in after Noise to both sides descendingDisplacement continuous generation Beat Sample of thin metal order of ininitial state driving of harmonic sound No. layers 12e thickness (μm)(μm) Delamination components generation II-1 None None 50.0 49.9 NoneNone None II-2 None None 50.0 49.9 None None None II-3 None None 55.054.9 None None None II-4 None None 55.0 54.9 None None None II-5Existence None 60.0 59.9 None None None II-6 Existence None 60.0 60.0None None None II-7 Existence None 60.0 60.0 None None None II-8Existence Existence 60.0 60.0 None None None II-9 None None 45.0 42.0Occurred Occurred Occurred

As apparent from Table 5, in Sample No. II-9 as a comparative example,the stress exerted on the stacking interface was concentrated at apoint, which increased load and caused delamination (inter-layerpeeling), as well as beat sound generation and noise generation. On theother hand, in Samples Nos. II-1 to II-8 as the present invention, noremarkable drop in the element displacement was confirmed even afterbeing continuously driven 1×10⁹ times, and hence these samples hadeffective displacements required for the piezoelectric actuators,respectively. This shows that the piezoelectric actuators havingexcellent durability were achieved.

Particularly, it can be seen that Sample No. II-3, in which the stressrelaxing layer (the thin metal layer 12 e) and the stress concentratinglayer (the thick metal layer 12 f) were adjacent to each other with thepiezoelectric layer 11 in between, were capable of increasing theelement displacement and also manufacturing the multilayer actuatorsexhibiting a stable element displacement. It can also be seen thatSamples Nos. II-4 to II-8, in which the stress relaxing layers wereinterposed with the piezoelectric layer 11 in between, were capable ofachieving the largest element displacement, and also manufacturing thepiezoelectric actuators which caused little change of the elementdisplacement and had extremely excellent durability, thereby exhibitinga stable element displacement. Among others, Samples Nos. II-6 and II-7,in which the stress relaxing layer (the thin metal layer 12 e) and thestress concentrating layer (the thick metal layer 12 f) were arranged onthe boundary with the inactive layer, had extremely excellentdurability.

Example II-b

Piezoelectric actuators were obtained (Samples Nos. II-10 to II-14 inTable 6) by changing the thickness of the metal layers 12 (Y2/X2 andZ2/X2) in the piezoelectric actuators of Sample No. II-8 in the aboveExample II-a, into those as shown in Table 6. As a comparative example,the piezoelectric actuator of Sample No. II-9 in Example II-a was alsopresented (Sample No. II-15 in Table 6). By applying a dc voltage of170V to each of the obtained multilayer piezoelectric elements, everypiezoelectric actuator had a displacement in the stacking direction.Since in the piezoelectric actuator of Sample No. II-15, the thicknessof all of the metal layers was set to about 5 μm, these are expressed asX2=5 μm, Y2=5 μm and Z2=5 μm, and the thickness ratio was expressed asY2/X2=1, and Z2/X2=1 in Table 6.

In the same manner as in Example I-a, a continuous driving test wasconducted for each of the obtained piezoelectric actuators (Samples Nos.II-10 to II-15 in Table 6). The results are shown in Table 6.

TABLE 6 Result of continuous driving test Displacement after NoiseConstitution Displacement continuous generation Sample of metal Y2 Z2 ininitial state driving of harmonic Beat sound No. layers X2 (μm) (μm)(μm) Y2/X2 Z2/X2 (μm) (μm) Delamination components generation II-10 FIG.11 5 0.5 10 0.1 2 60.0 60.0 None None None II-11 FIG. 11 5 1.5 7.5 0.31.5 65.0 65.0 None None None II-12 FIG. 11 5 2.5 6 0.5 1.2 70.0 70.0None None None II-13 FIG. 11 5 4 5.5 0.8 1.1 70.0 70.0 None None NoneII-14 FIG. 11 5 4.5 5.25 0.9 1.05 65.0 65.0 None None None II-15 FIG. 235 5 5 1 1 45.0 42.0 Occurred Occurred Occurred

As apparent from Table 6, in Sample No. II-15, in which Y2/X2 was largerthan 0.9, and Z2/X2 was smaller than 1.05, the stress exerted on thestacking interface was concentrated at a point, which increased load andcaused delamination (inter-layer peeling), as well as beat soundgeneration and noise generation.

On the other hand, Samples Nos. II-10 to II-14, in which Y2/X2 was inthe range of 0.1 to 0.9, and Z2/X2 was in the range of 1.05 to 2, werecapable of achieving the largest element displacement and also achievingthe multilayer actuators which caused little change of the elementdisplacement and had extremely excellent durability, thereby exhibitinga stable element displacement. In particular, Samples Nos. II-12 andII-13, in which Y2/X2 was in the range of 0.5 to 0.8, and Z2/X2 was inthe range of 1.1 to 1.2, were capable of achieving the multilayeractuators having excellent element displacement.

Example II-c

Piezoelectric actuators were obtained (Samples Nos. II-16 to II-33 inTable 7) by changing the material compositions of the metal layers 12 inthe piezoelectric actuators of Sample No. II-8 in the above ExampleII-a, into those as shown in Table 7. By applying a dc voltage of 170Vto each of the obtained multilayer piezoelectric elements, everypiezoelectric actuator had a displacement in the stacking direction.

In the same manner as in Example I-a, a continuous driving test wasconducted for each of the obtained piezoelectric actuators. Each rate ofchange of the displacement (%) was calculated in the same manner as inExample I-c. The results are shown in Table 7.

TABLE 7 Result of continuous Material compositions driving test of metallayers 12 Rate of change of Sample Pd Ag Cu Ni displacement No. (% bymass) (% by mass) (% by mass) (% by mass) (%) II-16 0.001 99.999 0 0 0.7II-17 0.01 99.99 0 0 0.7 II-18 0.1 99.9 0 0 0.4 II-19 0.5 99.5 0 0 0.2II-20 1 99 0 0 0.2 II-21 2 98 0 0 0 II-22 4 95 1 0 0 II-23 5 95 0 0 0II-24 8 92 0 0 0 II-25 9 91 0 0 0.2 II-26 9.5 90.5 0 0 0.2 II-27 10 90 00 0.4 II-28 15 85 0 0 0.7 II-29 0 0 100 0 0.2 II-30 0 0 99.9 0.1 0 II-310 0 0 100 0.4 II-32 20 80 0 0 0.9 II-33 30 70 0 0 0.9

As apparent from Table 7, in Sample Nos. II-32 and II-33, the content ofmetal of groups 8 to 10 in the metal composition in the metal layers 12was above 15% by mass, and the content of metal of group 11 was below85%. Consequently, the metal layers 12 had a large specific resistance,and when the multilayer piezoelectric element was continuously driven,heat was generated, and the displacement of the piezoelectric actuatorwas lowered.

On the other hand, Samples Nos. II-16 to II-28 were composed mainly of ametal composition satisfying the following relationship of: 0<M1≦15,85≦M2<100, M1+M2=100% by mass, where M1% by mass is a content of metalin groups 8 to 10 in the metal layers 12, and M2% by mass is a contentof a metal in group 11b. It was therefore capable of reducing thespecific resistance of the metal layers 12, and suppressing the heatgeneration occurred in the metal layers 12 even in the continuousdriving, thereby manufacturing the multilayer actuators with a stableelement displacement. It can also be seen that Samples Nos. II-29 to 31were capable of reducing the specific resistance of the metal layers 12,and suppressing the heat generation occurred in the metal layers 12 evenin the continuous driving, thereby manufacturing the multilayeractuators with a stable element displacement.

Example II-d

<Manufacturing of Piezoelectric Actuators>

Piezoelectric actuators composed of a multilayer piezoelectric elementwere manufactured as follows.

In the same manner as in Example II-a, there were prepared 30 sheets onwhich the respective metal layers were printed. Separately, green sheetsconstituting inactive layer 14 were prepared. These two kinds of sheetswere stacked so that 5 pieces of the inactive layers, 30 pieces ofstacked bodies and 5 pieces of the inactive layers were stacked in thisorder and from bottom to top, thereby obtaining a stacked matter.

The stacking was made in the combinations shown in Table 8. The detailsin Table 8 are as follows.

“Arrangement of the thin metal layers 12 e and the thick metal layers 12f” means that whether or not the thin metal layers 12 e and the thickmetal layers 12 f are oppositely arranged with at least one layer of thepiezoelectric layer 11 in between.

The stacked matter was pressed, debindered and sintered. In thesintering process, the stacked body was retained at 800° C. for twohours, and then sintered at 1000° C. for two hours, thereby obtaining astacked body 13. The metal filling rate of the metal layers 12 d to 12 fin the stacked body 13 were measured. The results are as follows.

The thickness X2 of the main metal layer 12 d was 5 μm.

The thickness Y2 of the thin metal layer 12 e was 2 μm.

The thickness Z2 of the thick metal layer 12 f was 7 μm.

Next, in the same manner as in Example I-a, each multilayerpiezoelectric element was obtained by forming external electrodes 15 onthe stacked body 13. Then, the polarization processing was performed byconnecting lead wires to the external electrodes 15 of the obtainedmultilayer piezoelectric element, respectively, and by applying throughthe lead wires a dc electric field of 3 kV/mm for 15 minutes to thepositive electrode and the negative electrode of the external electrodes15, respectively. Thus, each piezoelectric actuator using the multilayerpiezoelectric element as shown in FIG. 1 was manufactured (Sample Nos.II-34 to II-37 in Table 8). By applying a dc current of 170V to theobtained multilayer piezoelectric element, every piezoelectric actuatorhad a displacement in the stacking direction.

<Evaluations>

In the same manner as in Example I-a, a continuous driving test wasconducted for each of the obtained piezoelectric actuators. Theevaluation results are shown in Table 8.

TABLE 8 Result of continuous driving test Opposite Displacementarrangement of after Existence of thin Existence of thick thin metallayers Displacement continuous Sample metal layers 12e metal layers 12f12e and thick in initial state driving No. (Position) (Position) metallayers 12f (μm) (μm) Delamination II-34 Existence None None 5.0 4.9 None1.30 II-35 None Existence None 5.5 5.4 None 1.30 II-36 ExistenceExistence Existence 5.5 5.5 None 1.30 2.29 II-37 None None None 5.0 4.2Occurred

As apparent from Table 8, in Sample No. II-37 as a comparative example,the stress exerted on the stacking interface was concentrated at apoint, which increased load and caused delamination (inter-layerpeeling), as well as beat sound generation and noise generation. On theother hand, in Samples Nos. II-34 to II-36 as the present invention, noremarkable drop in the element displacement was confirmed even afterbeing continuously driven 1×10⁹ times, and hence these samples hadeffective displacements required for the piezoelectric actuators,respectively. This shows that the piezoelectric actuators havingexcellent durability were achieved.

Particularly, it can be seen that Sample No. II-36, in which the stressrelaxing layer (the thin metal layer 12 e) and the stress concentratinglayer (the thick metal layer 12 f) were adjacent to each other with thepiezoelectric layer 11 in between, were capable of increasing theelement displacement and also manufacturing the multilayer actuatorsexhibiting a stable element displacement.

Example III-a

<Manufacturing of Piezoelectric Actuators>

Piezoelectric actuators composed of the multilayer piezoelectric elementaccording to the ninth preferred embodiment were manufactured asfollows.

Firstly, in the same manner as in Example I-a, a ceramic green sheethaving a thickness of 150 μm and constituting a piezoelectric layer 11was prepared. On one surface of this ceramic green sheet, 300 pieces ofsheets were stacked which were formed by screen printing using aconductive paste. The conductive paste was prepared by adding binder toan alloy composed mainly of silver-palladium so as to have thecomposition as shown in Table 9. This was sintered to obtain a stackedbody 13, under the sintering condition that the stacked matter wasretained at 800° C. for two hours, and then sintered at 1000° C. for twohours.

At this time, a conductive paste obtained by adding binder tosilver-palladium alloy was printed on the portions for forming ahigh-ratio metal layer 12 h so that they had a thickness of 3 μm and hadthe composition as shown in Table 9, and the high-ratio metal layers 12h were arranged on the 50th layer, the 100th layer, the 150th layer, the200th layer and 250th layer, respectively.

Next, in the same manner as in Example I-a, each multilayerpiezoelectric element was obtained by forming external electrodes 15 onthe stacked body 13. Then, the polarization processing was performed byconnecting lead wires to the external electrodes 15 of the obtainedmultilayer piezoelectric element, respectively, and by applying throughthe lead wires a dc electric field of 3 kV/mm for 15 minutes to thepositive electrode and the negative electrode of the external electrodes15, respectively. Thus, each piezoelectric actuator using the multilayerpiezoelectric element as shown in FIG. 1 was manufactured (Sample Nos.III-1 to III-6 in Table 9). By applying a dc current of 170V to theobtained multilayer piezoelectric element, every piezoelectric actuatorhad a displacement in the stacking direction.

<Evaluations>

In the same manner as in Example I-a, a continuous driving test wasconducted for each of the obtained piezoelectric actuators. Theevaluation results are shown in Table 9. The metal layers other than thehigh-ratio metal layer had substantially the same composition as shownin Table 9.

TABLE 9 Result of continuous driving test Main Composition CompositionDisplacement Consti- compo- High-ratio of high- of other Displacementafter Noise tution nents metal ratio metal metal in initial continuousgeneration of Sample of metal of metal compo- layers 12h layers 12gstate driving harmonic Beat sound No. layers layers 12 nents (% by mass)(% by mass) (μm) (μm) Delamination components generation III-1 FIG. 14Ag—Pd Ag Ag70% Ag65% 50.0 49.9 None None None Alloy Pd30% Pd30% Pt5%III-2 FIG. 14 Ag—Pd Ag Ag75% Ag70% 50.0 49.9 None None None Alloy Pd25%Pd30% III-3 FIG. 14 Ag—Pd Ag Ag85% Ag80% 55.0 54.9 None None None AlloyPd15% Pd20% III-4 FIG. 14 Ag—Pd Ag Ag90% Ag85% 60.0 59.9 None None NoneAlloy Pd10% Pd15% III-5 FIG. 14 Ag—Pd Ag Ag95% Ag90% 60.0 60.0 None NoneNone Alloy Pd5% Pd10% III-6 FIG. 23 Ag—Pd X Ag70% Ag70% 45.0 42.0Occurred Occurred Occurred Alloy Pd30% Pd30%

As apparent from Table 9, in Sample No. III-6 as a comparative example,the stress exerted on the stacking interface was concentrated at apoint, which increased load and caused peeling, as well as beat soundgeneration and noise generation. On the other hand, in Samples Nos.III-1 to III-5 as the present invention, no remarkable drop in theelement displacement was confirmed even after being continuously driven1×10⁹ times, and hence these samples had effective displacementsrequired for the piezoelectric actuators, respectively. That is, thepiezoelectric actuators having excellent durability were achieved.

Example III-b

<Manufacturing of Piezoelectric Actuators>

Piezoelectric actuators composed of the multilayer piezoelectric elementaccording to the tenth preferred embodiment were manufactured asfollows.

Firstly, in the same manner as in Example I-a, a ceramic green sheethaving a thickness of 150 μm and constituting a piezoelectric layer 11was prepared. On one surface of this ceramic green sheet, 300 pieces ofsheets were stacked which were formed by screen printing using aconductive paste. The conductive paste was prepared by adding binder tosilver-palladium alloy so as to have the composition as shown in Table10. This was sintered to obtain a stacked body 13, under the sinteringcondition that the stacked matter was retained at 800° C., and thensintered at 1000° C.

At this time, a conductive paste consisting of 100% of silver wasprinted on the portion for forming a high-ratio metal layer 12 j so thatthey had a thickness of 3 μm, and the high-ratio metal layers 12 j werearranged on the 50th layer, the 100th layer, the 150th layer, the 200thlayer and 250th layer, respectively.

Next, in the same manner as in Example I-a, each multilayerpiezoelectric element was obtained by forming external electrodes 15 onthe stacked body 13. Then, the polarization processing was performed byconnecting lead wires to the external electrodes 15 of the obtainedmultilayer piezoelectric element, respectively, and by applying throughthe lead wires a do electric field of 3 kV/mm for 15 minutes to thepositive electrode and the negative electrode of the external electrodes15, respectively. Thus, each piezoelectric actuator using the multilayerpiezoelectric element as shown in FIG. 1 was manufactured (Sample Nos.III-7 to III-12 in Table 10). By applying a dc current of 170V to theobtained multilayer piezoelectric element, every piezoelectric actuatorhad a displacement in the stacking direction.

<Evaluations>

In the same manner as in Example I-a, a continuous driving test wasconducted for each of the obtained piezoelectric actuators. Theevaluation results are shown in Table 10. The metal layers other thanthe high-ratio metal layer had substantially the same composition asshown in Table 10.

TABLE 10 Result of continuous driving test Main Composition DisplacementConsti- compo- High-ratio of high-ratio Composition after Noise tutionnents metal metal layers other metal Displacement continuous generationof Sample of metal of metal compo- 12j layers 12i in initial statedriving harmonic Beat sound No. layers layers 12 nents (% by mass) (% bymass) (μm) (μm) Delamination components generation III-7 FIG. 15 Ag AgAg100% Ag65% 50.0 49.9 None None None Pd30% Pt5% III-8 FIG. 15 Ag AgAg100% Ag70% 50.0 49.9 None None None Pd30% III-9 FIG. 15 Ag Ag Ag100%Ag80% 55.0 54.9 None None None Pd20% III-10 FIG. 15 Ag Ag Ag100% Ag85%60.0 59.9 None None None Pd15% III-11 FIG. 15 Ag Ag Ag100% Ag95% 60.060.0 None None None Pd5% III-12 FIG. 23 Ag X Ag70% Ag70% 45.0 42.0Occurred Occurred Occurred Pd30% Pd30%

As shown in Table 10, in Sample No. III-12 as a comparative example, thestress exerted on the stacking interface was concentrated at a point,which increased load and caused peeling, as well as beat soundgeneration and noise generation. On the other hand, in Samples Nos.III-7 to III-11 as the preferred embodiments of the present invention,no remarkable drop in the element displacement was confirmed even afterbeing continuously driven 1×10⁹ times, and hence these samples hadeffective displacements required for the piezoelectric actuators,respectively. That is, the piezoelectric actuators having excellentdurability were achieved.

Example III-c

<Manufacturing of Piezoelectric Actuators>

Piezoelectric actuators composed of the multilayer piezoelectric elementaccording to the eleventh preferred embodiment were manufactured asfollows.

Firstly, in the same manner as in Example I-a, a ceramic green sheethaving a thickness of 150 μm and constituting a piezoelectric layer 11was prepared. On one surface of this ceramic green sheet, 300 pieces ofsheets were stacked which were formed by screen printing using aconductive paste prepared by adding binder to copper powder. This wassintered in nitrogen atmosphere, thereby obtaining a stacked body 13,under the sintering condition that the stacked matter was retained at800° C., and then sintered at 1000° C.

At this time, a conductive paste consisting of silver-palladium alloy asshown in Table 11 was printed on the portions for forming a metal layer12 l so that they had a thickness of 3 μm, and the metal layers 12 lwere arranged on the 50th layer, the 100th layer, the 150th layer, the200th layer and 250th layer, respectively.

Next, in the same manner as in Example I-a, each multilayerpiezoelectric element was obtained by forming external electrodes 15 onthe stacked body 13. Then, the polarization processing was performed byconnecting lead wires to the external electrodes 15 of the obtainedmultilayer piezoelectric element, respectively, and by applying throughthe lead wires a dc electric field of 3 kV/mm for 15 minutes to thepositive electrode and the negative electrode of the external electrodes15, respectively. Thus, each piezoelectric actuator using the multilayerpiezoelectric element as shown in FIG. 1 was manufactured (Sample Nos.III-13 to III-19 in Table 11). By applying a dc current of 170V to theobtained multilayer piezoelectric element, every piezoelectric actuatorhad a displacement in the stacking direction.

<Evaluations>

In the same manner as in Example I-a, a continuous driving test wasconducted for each of the obtained piezoelectric actuators. Theevaluation results are shown in Table 11. All of the metal layers havingthe same main component had substantially the same composition as shownin Table 11.

TABLE 11 Result of continuous driving test Displacement Composition ofafter Noise Constitution Composition of metal layers Displacementcontinuous generation of Sample of metal metal layers 12k 12l in initialstate driving harmonic Beat sound No. layers (% by mass) (% by mass)(μm) (μm) Delamination components generation III-13 FIG. 16 Cu100% Ag65%50.0 49.9 None None None Pd30% Pt5% III-14 FIG. 16 Cu100% Ag70% 50.049.9 None None None Pd30% III-15 FIG. 16 Cu100% Ag80% 55.0 54.9 NoneNone None Pd20% III-16 FIG. 16 Cu100% Ag85% 60.0 59.9 None None NonePd15% III-17 FIG. 16 Cu100% Ag95% 60.0 60.0 None None None Pd5% III-18FIG. 23 Ag70% Ag70% 45.0 42.0 Occurred Occurred Occurred Pd30% Pd30%III-19 FIG. 23 Cu100% Cu100% 45.0 42.0 Occurred Occurred Occurred

As shown in Table 11, in Samples Nos. 111-18 and III-19 as comparativeexamples, the stress exerted on the stacking interface was concentratedat a point, which increased load and caused peeling, as well as beatsound generation and noise generation. On the other hand, in SamplesNos. III-13 to III-17 as the preferred embodiments of the presentinvention, no remarkable drop in the element displacement was confirmedeven after being continuously driven 1×10⁹ times, and hence thesesamples had effective displacements required for the piezoelectricactuators, respectively. That is, the piezoelectric actuators havingexcellent durability were achieved.

Example III-d

The material compositions of the metal layers 12 in the piezoelectricactuators of Sample No. III-5 in the above Example III-a was changed,and each rate of change of the displacement (%) was calculated in thesame manner as in Example I-c. The results are shown in Table 12.

TABLE 12 Pd composition Pd composition Ag composition of Ag compositionof of other metal of other metal high-ratio metal high-ratio metal Rateof change of Sample layers 12g layers 12g layers 12h layers 12hdisplacement No. (% by mass) (% by mass) (% by mass) (% by mass) (%)III-20 0.01 99.99 0.001 99.999 0.7 III-21 0.1 99.9 0.01 99.99 0.4 III-220.5 99.5 0.1 99.9 0.2 III-23 1 99 0.5 99.5 0.2 III-24 2 98 1 99 0.1III-25 4 96 2 98 0 III-26 5 95 4 96 0 III-27 8 92 5 95 0 III-28 9 91 892 0.1 III-29 9.5 90.5 9 91 0.2 III-30 10 90 9.5 90.5 0.4 III-31 15 8510 90 0.7 III-32 20 80 15 58 0.9 III-33 30 70 20 80 0.9 III-34 0 100 0100 Damaged by migration

As apparent from Table 12, in Sample Nos. III-34, in which all of themetal layers 12 were composed of 100% silver, ion migration of silveroccurred, and the multilayer piezoelectric element was broken, thecontinuous driving was unable. In Samples Nos. 111-32 and III-33, inwhich the palladium content in the metal composition of the metal layers12 was above 15 mass %, and the silver content was below 85% by mass,the metal layers 12 had a large specific resistance, and when themultilayer piezoelectric element was continuously driven, heat wasgenerated, and the displacement of the piezoelectric actuator waslowered.

On the other hand, Samples Nos. III-20 to III-31 were composed mainly ofa metal composition satisfying the following relationship of: 0<M1≦15,85≦M2<100, M1+M2=100% by mass, where M1% by mass is a content of metalin groups 8 to 10 in the metal layers 12, and M2% by mass is a contentof a metal in group 1b. It was therefore capable of reducing thespecific resistance of the metal layers 12, and suppressing the heatgeneration occurred in the metal layers 12 even in the continuousdriving, thereby manufacturing the multilayer actuators with a stableelement displacement.

Particularly, Samples Nos. 111-25 to 27 were composed mainly of a metalcomposition satisfying the following relationship of: 2≦M1≦8, 92≦M2≦98,M1+M2=100% by mass, where M1% by mass is a content of metal in groups 8to 10 in the metal layers 12, and M2% by mass is a content of a metal ingroup 1b. It was therefore capable of reducing the specific resistanceof the metal layers 12, and suppressing the heat generation occurred inthe metal layers 12 even in the continuous driving, therebymanufacturing the multilayer actuators which had no change in theelement displacement and hence were extremely stable.

Example III-e

From the multilayer piezoelectric elements shown in Table 9, a piece ofeach sample was taken out, and processed in 3 mm×4 mm×36 mm so that theelectrode surface of the metal layers 12 was substantially perpendicularto the longitudinal direction of the specimen. Then, the bendingstrength was measured under the four-point bending in JIS R 1601. Atthis time, the location of breakage was confirmed to specify a weakadhesion portion of the multilayer piezoelectric element.

That is, the breakage within the piezoelectric layer 11 shows that thestrength of the piezoelectric body is low. The breakage within the metallayer 12 shows that the strength of the metal layer 12 is low. Thebreakage on the interface between the piezoelectric layer 11 and themetal layer 12 shows that the strength of the interface between thepiezoelectric layer 11 and the metal layer 12 is low. The results areshown in Table 13. Although the durability when each sample wasfunctioned as an actuator is already presented in Table 9, this is alsopresented for reference in Table 13.

TABLE 13 Composition of Composition of Main High-ratio high-ratio metalother metal layer Sample Constitution of component of metal layer 12h12g No. metal layers metal layers 12 component (% by mass) (% by mass)Breakage Delamination III-1 FIG. 14 Ag—Pd Ag Ag70% Ag65% High-ratiometal None Alloy Pd30% Pd30% Pt5% III-2 FIG. 14 Ag—Pd Ag Ag75% Ag70%High-ratio metal None Alloy Pd25% Pd30% III-3 FIG. 14 Ag—Pd Ag Ag85%Ag80% High-ratio metal None Alloy Pd15% Pd20% III-4 FIG. 14 Ag—Pd AgAg90% Ag85% High-ratio metal None Alloy Pd10% Pd15% III-5 FIG. 14 Ag—PdAg Ag95% Ag90% High-ratio metal None Alloy Pd5% Pd10% III-6 FIG. 23Ag—Pd x Ag70% Ag70% Piezoelectrics Occurred Alloy Pd30% Pd30%

In Sample No. III-6 as a comparative example, a breakage occurred in thepiezoelectric layer 11. This shows that all of the piezoelectric layers11 and the metal layers 12 were connected to each other at a highstrength. From this, when this sample was continuously driven 1×10⁹times as an actuator, the stress exerted on the stacking interface wasconcentrated at a point, which increased load and caused peeling.

On the other hand, in Sample No. III-1 to III-5 as the preferredembodiment of the present invention, a breakage occurred on theinterface between the piezoelectric layer 11 and the high-ratio metallayer. This shows that the adhesion between the high-ratio metal layerand the piezoelectric layer was the lowest. From this, it can beconsidered as follows. That is, when stress was exerted during thecontinuous driving, the high-ratio metal layer having low adhesion wasdeformed to generate the phenomenon for relaxing the stress. Even afterbeing continuously driven 1×10⁹ times as an actuator, no peelingoccurred, exhibiting excellent durability.

Example III-f

From the multilayer piezoelectric elements shown in Table 9, a piece ofeach sample was taken out, and the Vickers hardness was measured byusing a Micro Vickers Tester such as Model MVK-H3 manufactured by AkashiSeisakusho Co., Ltd. In the measurements, in order to avoid theinfluence of the piezoelectric layer 11 as a base, there was used themethod in which a diamond probe was forced into the metal layer 12 froma direction perpendicular to the stacking direction of the metal layers12. The results are shown in Table 14. Although the durability when eachsample was functioned as an actuator is already presented in Table 9,this is also presented for reference in Table 14.

TABLE 14 Composition of Composition of Constitution Main High-ratiohigh-ratio metal Vickers hardness other metal layers Vickers hardnessSample of metal component of metal layers 12h of high-ratio metal 12g ofother metal No. layers metal layers 12 component (% by mass) layers 12h(% by mass) layers 12g Delamination III-1 FIG. 14 Ag—Pd Ag Ag70% 35Ag65% 40 None Alloy Pd30% Pd30% Pt5% III-2 FIG. 14 Ag—Pd Ag Ag75% 32Ag70% 35 None Alloy Pd25% Pd30% III-3 FIG. 14 Ag—Pd Ag Ag85% 29 Ag80% 31None Alloy Pd15% Pd20% III-4 FIG. 14 Ag—Pd Ag Ag90% 27 Ag85% 29 NoneAlloy Pd10% Pd15% III-5 FIG. 14 Ag—Pd Ag Ag95% 25 Ag90% 27 None AlloyPd5% Pd10% III-6 FIG. 23 Ag—Pd x Ag70% 35 Ag70% 35 Occurred Alloy Pd30%Pd30%

In Sample No. III-6 as a comparative example, all of the metal layershad the same composition and hence had the same hardness. This showsthat all of the piezoelectric layers 11 are connected with the metallayer of the same hardness. When Sample No. III-6 was continuouslydriven 1×10⁹ times as an actuator, the stress exerted on the stackinginterface was concentrated at a point, which increased load and causedpeeling.

On the other hand, Sample No. III-1 to III-5 as the preferred embodimentof the present invention had superior results to other metal layers interms of the hardness of the high-ratio metal layer. This shows that thehigh-ratio metal layer was softer than other metal layers. From this, itcan be considered as follows. That is, when stress was exerted duringthe continuous driving, the high-ratio metal layer being soft wasdeformed to generate the phenomenon for relaxing the stress. Even afterbeing continuously driven 1×10⁹ times as an actuator, no peelingoccurred, exhibiting excellent durability.

Example III-g

<Manufacturing of Piezoelectric Actuators>

Piezoelectric actuators composed of the multilayer piezoelectric elementhaving a tilted concentration part were manufactured as follows.

Firstly, in the same manner as in Example I-a, a ceramic green sheethaving a thickness of 150 μm and constituting a piezoelectric layer 11was prepared. On one surface of this ceramic green sheet, 300 pieces ofsheets were stacked which were formed by screen printing using aconductive paste prepared by adding binder to silver-palladium alloy(80% by mass of silver and 20% by mass of palladium). This was sinteredto obtain a stacked body 13, under the sintering condition that thestacked matter was retained at 800° C. for two hours, and then sinteredat 1000° C. for two hours.

At this time, a conductive paste of silver-palladium alloy (85% by massof silver and 15% by mass of palladium) was printed on the portions forforming a high-ratio metal layer 12 h so that they had a thickness of 3μm, and further that the high-ratio metal layers 12 h were arranged onthe 50th layer, the 100th layer, the 150th layer, the 200th layer and250th layer, respectively, and the silver concentration was graduallyreduced from the high-ratio metal layer 12 h, as shown in FIG. 17.

Next, in the same manner as in Example I-a, each multilayerpiezoelectric element was obtained by forming external electrodes 15 onthe stacked body 13. Then, the polarization processing was performed byconnecting lead wires to the external electrodes 15 of the obtainedmultilayer piezoelectric element, respectively, and by applying throughthe lead wires a dc electric field of 3 kV/mm for 15 minutes to thepositive electrode and the negative electrode of the external electrodes15, respectively. Thus, each piezoelectric actuator using the multilayerpiezoelectric element as shown in FIG. 1 was manufactured. By applying adc current of 170V to the obtained multilayer piezoelectric element,every piezoelectric actuator had a displacement in the stackingdirection.

<Evaluations>

In the same manner as in Example I-a, a continuous driving test wasconducted for each of the obtained piezoelectric actuators. Each rate ofchange of the displacement (%) was calculated in the same manner as inExample I-c. The evaluation results are shown in Table 15.

TABLE 15 Main Composition of Composition of Existence of Constitutioncomponent of High-ratio high-ratio metal other metal tilted Sample ofmetal metal layers metal layers 12h layers 12g concentration No. layers12 component (% by mass) (% by mass) part III-35 FIG. 14 Ag—Pd Ag Ag85%Ag80% Existence Alloy Pd15% Pd20% III-36 FIG. 14 Ag—Pd Ag Ag85% Ag80%None Alloy Pd15% Pd20% III-37 FIG. 23 Ag—Pd x Ag70% Ag70% ExistenceAlloy Pd30% Pd30% Result of continuous driving test Displacement afterRate of Noise Displacement continuous change of generation of Sample ininitial state driving displacement harmonic Beat sound No. (μm) (μm) (%)Delamination components generation III-35 54.9 54.9 0.0 None None NoneIII-36 55.0 54.9 0.2 None None None III-37 45.0 42.0 6.7 OccurredOccurred Occurred

As shown in Table 15, in Sample No. II-37 as a comparative example, thestress exerted on the stacking interface was concentrated at a point,which increased load and caused peeling, as well as beat soundgeneration and noise generation. On the other hand, Samples Nos. III-35and III-36 as the preferred embodiments of the present invention hadgood results. Unlike Sample No. III-36, particularly, in Sample No.III-35, no drop in the element displacement was confirmed even afterbeing continuously driven 1×10⁹ times, and hence had effectivedisplacement required for the piezoelectric actuator. That is, thepiezoelectric actuators having extremely excellent durability wasachieved.

Example IV

<Manufacturing of Piezoelectric Actuators>

Piezoelectric actuators composed of the multilayer piezoelectric elementaccording to the twelfth preferred embodiment were manufactured asfollows.

Firstly, in the same manner as in Example I-a, a plurality of ceramicgreen sheets having a thickness of 150 μm and constituting apiezoelectric layer 11 were prepared. On one surface of each of theseceramic green sheets, a conductive paste prepared by adding binder tosilver-palladium alloy (95% by mass of silver and 5% by mass ofpalladium) was printed by screen printing. In this manner, there wereprepared 300 sheets on which the conductive paste was printed.Separately, green sheets serving as protecting layers was prepared.These layers were stacked so that 30 pieces of the protecting layers,300 pieces of stacked bodies and 30 pieces of the protecting layers werearranged in this order and from bottom to top. This stacked matter waspressed, debindered and sintered to obtain a stacked body 13, under thesintering conditions that the stacked matter was retained at 800° C. fortwo hours, and then sintered at 1000° C. for two hours.

At this time, a conductive paste prepared by adding binder tosilver-palladium alloy (95% by mass of silver and 5% by mass ofpalladium) was printed on the portions for forming other metal layers sothat they had a thickness of 5 or 10 μm after sintering. Depending onthe case, voids were formed in the metal layers by adding acryl beads of0.2 μm to the above conductive paste. A conductive paste, which wasprepared by adding a proper amount of acryl beads having a mean particlesize of 0.2 μm, and binder to particles of silver-palladium alloy (95%by mass of silver and 5% by mass of palladium) whose surfaces wereoxidized, was printed on the portions for forming other metal layers sothat they had a thickness of 1 to 4 μm after sintering. In this manner,the void ratio as shown in Table 16 were attained.

Further, high-resistance metal layers 12 m in the stacked body 13 hadthe layer number as shown in Table 16. The high-resistance metal layers12 m, except for those of Sample No. IV-9, were regularly arranged.Specifically, in Sample No. IV-1 that was one in the layer number of thehigh-resistance metal layers, the high-resistance metal layer wasarranged on the 150th layer from the top of the stacked body. In SampleNo. IV-2 that was two in the layer number of the high-resistance metallayers, the high-resistance metal layers were regularly arranged on the100th layer and 200th layer from the top of the stacked body. In SampleNo. IV-3 that was three in the layer number of the high-resistance metallayers, the high-resistance metal layers were regularly arranged atintervals of 50 layers from the top of the stacked body.

In the sample that was 14 in the layer number of the high-resistancemetal layers, the high-resistance metal layers were regularly arrangedat intervals of 20 layers, and the sample that was 59 in the layernumber of the high-resistance metal layers, the high-resistance metallayers were regularly arranged at intervals of 5 layers. In the samplethat was 10 in the layer number of the high-resistance metal layers, thehigh-resistance metal layers were regularly arranged at intervals of 26,27, 27, 28, 28, 28, 28, 28, 27 and 27. In the sample that was 39 in thelayer number of the high-resistance metal layers, the high-resistancemetal layers were regularly arranged by alternately taking the intervalsof seven layers and eight layers from the top of the stacked body, suchas 7, 8, 7 and 8. In the sample that was 20 in the layer number of thehigh-resistance metal layers, the high-resistance metal layers wereregularly arranged at the intervals of 13, 13, 13, 13, 14, 14, 15, 15,16, 16, 16, 16, 16, 15, 15, 14, 14, 13, 13 and 13. In Sample No. IV-9,which was the samples that was 20 in the layer number of thehigh-resistance metal layers, and the high-resistance metal layers werenot regularly arranged, the high-resistance metal layers were arrangedfrom the top of the stacked body at the intervals of 5, 5, 25, 25, 15,10, 20, 20, 10, 10, 10, 10, 10, 20, 20, 10, 15, 25, 25 and 5.

As the high-resistance metal layers 12 m, depending on the case, therewere prepared those containing a high-resistance component such as PZT,lead titanate, alumina, titania, silicon nitride, silica or the like.

Next, in the same manner as in Example I-a, each multilayerpiezoelectric element was obtained by forming external electrodes 15 onthe stacked body 13. Then, the polarization processing was performed byconnecting lead wires to the external electrodes 15 of the obtainedmultilayer piezoelectric element, respectively, and by applying throughthe lead wires a dc electric field of 3 kV/mm for 15 minutes to thepositive electrode and the negative electrode of the external electrodes15, respectively. Thus, each piezoelectric actuator using the multilayerpiezoelectric element as shown in FIG. 18 was manufactured (Samples Nos.IV-1 to IV-32 in Table 16). By applying a dc current of 170V to theobtained-multilayer piezoelectric element, every piezoelectric actuatorhad a displacement in the stacking direction.

<Evaluations>

In a test, each piezoelectric actuator was continuously driven up to2×10⁹ times by applying an alternating voltage of 0 to +170V at afrequency of 300 Hz at room temperature. The test was conducted using100 pieces per sample. After the test, the rate of the fractured sampleswas calculated, and the results were presented in Table 16, as afracture rate after testing. Further, using a metal microscope and anSEM or the like, the stacked portions were observed to confirm thenumber of layers with peeling. Further, the absolute value of adifference between the displacement of the multilayer piezoelectricelement at the initial stage and the displacement of the multilayerpiezoelectric element after the test, is divided by the displacement ofthe multilayer piezoelectric element at the initial stage, and theresult was multiplied by 100. The obtained value was presented in Table16, as a rate of change in the displacements before and after thedriving test. The results are shown in Table 16.

TABLE 16 Void ratio Content of high Content of high Number of of high-resistance resistance high- Arrangement resistance Void ratio ofcomponent in component in resistance of high- metal other metalhigh-resistance other metal Sample metal resistance layers layers metallayers layers No. layers metal layers (%) (%) (%) (%) IV-1 1 — 70 1 1 1IV-2 2 Almost regular 70 1 1 1 IV-3 5 Almost regular 70 1 1 1 IV-4 10Almost regular 70 1 1 1 IV-5 14 Almost regular 70 1 1 1 IV-6 20 Almostregular 70 1 1 1 IV-7 39 Almost regular 70 1 1 1 IV-8 59 Almost regular70 1 1 1 IV-9 20 irregular 70 1 1 1 IV-10 20 Almost regular 30 1 1 1IV-11 20 Almost regular 40 1 1 1 IV-12 20 Almost regular 50 1 1 1 IV-1320 Almost regular 90 1 1 1 IV-14 20 Almost regular 99 1 1 1 IV-15 20Almost regular 70 5 1 1 IV-16 20 Almost regular 70 10 1 1 IV-17 20Almost regular 1 1 40 1 IV-18 20 Almost regular 1 1 50 1 IV-19 20 Almostregular 1 1 70 1 IV-20 20 Almost regular 1 1 90 1 IV-21 20 Almostregular 1 1 99 1 IV-22 20 Almost regular 1 1 70 1 IV-23 20 Almostregular 1 1 70 1 IV-24 20 Almost regular 1 1 70 1 IV-25 20 Almostregular 1 1 70 1 IV-26 20 Almost regular 1 1 70 1 IV-27 20 Almostregular 70 1 1 1 IV-28 20 Almost regular 70 1 1 1 IV-29 20 Almostregular 70 1 1 1 IV-30 20 Almost regular 70 1 1 1 IV-31 20 Almostregular 70 1 1 1 IV-32 20 Almost regular 70 1 1 1 Thickness Changingrate of Thickness of high- displacement of other resistance Fracturebefore and after matal matal rate after continuous Sample Highresistance layers layers test driving test No. component (μm) (μm) (%)(%) IV-1 PZT 3 5 10 20 IV-2 PZT 3 5 3 10 IV-3 PZT 3 5 3 5 IV-4 PZT 3 5 03 IV-5 PZT 3 5 0 2.5 IV-6 PZT 3 5 0 1.5 IV-7 PZT 3 5 0 1.4 IV-8 PZT 3 50 1.3 IV-9 PZT 3 5 2 2.2 IV-10 PZT 3 5 0 2 IV-11 PZT 3 5 0 1.8 IV-12 PZT3 5 0 1.6 IV-13 PZT 3 5 0 1.6 IV-14 PZT 3 5 0 1.8 IV-15 PZT 3 5 0 1.6IV-16 PZT 3 5 0 1.6 IV-17 PZT 3 5 0 0.9 IV-18 PZT 3 5 0 0.5 IV-19 PZT 35 0 0.4 IV-20 PZT 3 5 0 0.5 IV-21 PZT 3 5 0 0.8 IV-22 Lead titanate 3 50 0.5 IV-23 Alumina 3 5 0 0.5 IV-24 Titania 3 5 0 0.6 IV-25 Siliconnitride 3 5 0 0.8 IV-26 Silica 3 5 0 0.8 IV-27 PZT 6 5 0 1.9 IV-28 PZT 45 0 1.6 IV-29 PZT 3 5 0 1.5 IV-30 PZT 2 5 0 1.4 IV-31 PZT 1 5 0 1.4IV-32 PZT 3 10 0 1.4

From Table 16, Sample No. IV-1 as a comparative example, in which thenumber of the high-resistance metal layer in the multilayerpiezoelectric element was one, could not disperse stress suitably, andthe generated crack was extended to the entire element, so that thefracture rate after the test was as large as 10%. The number of layerswhich caused peeling in the layers of the stacked body was as much as100. Further, the range of change in the displacement before and afterthe driving test was as large as 20%, and the durability was low.

On the other hand, in Sample Nos. IV-2 to IV-32 as the preferredembodiments of the present invention, the fracture rate after the 2×10⁹times of continuous driving was not more than 3%, and was extremelysuperior in durability to Sample Nos. IV-1 as a comparative example. Thesamples where the high-resistance metal layers were regularly arranged,for example, Sample No. IV-6, was superior in durability to Sample No.IV-9 where these layers were not regularly arranged, owing to theabsence of the fracture after the test, and a small rate of change inthe displacement before and after the test.

In Samples Nos. IV-10 to IV-16 in which 20 layers of the high-resistancemetal layers were regularly arranged, and the void ratio of thehigh-resistance metal layers was larger than that of other metal layers,the rate of change in the displacement before and after the test was assmall as 2.0% or below. This shows these samples had excellentdurability as the multilayer piezoelectric element. In Samples No. IV-11to IV-16 in which the void ratio of the high-resistance metal layers was40 to 99%, the rate of change in the displacement before and after thetest was smaller, namely 1.8% or below, exhibiting excellent durability.

In Samples Nos. IV-17 to IV-26 in which 20 layers of the high-resistancemetal layers were regularly arranged, and the content of the highresistance component in the high-resistance metal layers was larger thanthat of other metal layers, any one of these samples was not fractured,and the rate of change in the displacement before and after the test wasas remarkably small, namely 0.4% to 0.9%. This shows these samples hadexcellent durability as the multilayer piezoelectric element. Thesamples using PZT, lead titanate, alumina, or titania were furtherexcellent in durability.

In Samples Nos. IV-6, IV-28 to IV-32, in which the high-resistance metallayers and other metal layers had different thicknesses in order toconfirm the effect produced by setting so that the high-resistance metallayer had a smaller thickness than other metal layers, the rate ofchange in the displacement before and after the test was as small as1.6%, than Sample No. IV-27 in which the high-resistance metal layer hada larger thickness than other metal layers. This shows these samples hadexcellent durability.

Further, in the sample in which the electrical resistance ratio of thehigh-resistance metal layers to the piezoelectric layers is 1/10 to 1000times, and in the sample in which the electrical resistance ratio of thehigh-resistance metal layers to other metal layers is above 1000 times,no peeling occurred in the high-resistance metal layers, exhibitingexcellent durability.

From the foregoing results, the injectors containing the multilayerpiezoelectric elements of the present preferred embodiments,respectively, permit efficient injection and exhibit excellentdurability. Hence, these injectors become environmental friendlyproducts.

Example V

<Manufacturing of Piezoelectric Actuators>

Piezoelectric actuators composed of the multilayer piezoelectric elementaccording to the thirteenth preferred embodiment were manufactured asfollows.

Firstly, in the same manner as in Example I-a, a ceramic green sheethaving a thickness of 150 μm and constituting a piezoelectric layer 11was prepared. On one surface of this ceramic green sheet, 300 pieces ofsheets were stacked which were formed by screen printing using aconductive paste prepared by adding binder to silver-palladium alloy(95% by mass of silver and 5% by mass of palladium). This was sinteredto obtain a stacked body 13, under the sintering condition that thestacked matter was retained at 800° C., and then sintered at 1000° C.

At this time, printing was performed on the portions for forming metallayers so that they had a thickness of 10 μm by a process using a resistthickness of 20 μm, and printing was performed on the portions forforming metal parts so that they had a thickness of 5 μm by a processusing a resist thickness of 10 μm. The metal parts were arranged on the50th layer, the 100th layer, the 150th layer, the 200th layer and 250thlayer, respectively. In the metal layers composed of the metal parts,six metal parts were arranged as shown in FIG. 19( b).

Next, in the same manner as in Example I-a, each multilayerpiezoelectric element was obtained by forming external electrodes 15 onthe stacked body 13. Then, the polarization processing was performed byconnecting lead wires to the external electrodes 15 of the obtainedmultilayer piezoelectric element, respectively, and by applying throughthe lead wires a dc electric field of 3 kV/mm for 15 minutes to thepositive electrode and the negative electrode of the external electrodes15, respectively. Thus, each piezoelectric actuator using the multilayerpiezoelectric element as shown in FIG. 19 was manufactured (Sample Nos.V-1 to V-6 in Table 17). By applying a dc current of 170V to theobtained multilayer piezoelectric element, every piezoelectric actuatorhad a displacement in the stacking direction.

<Evaluations>

In the same manner as in Example I-a, a continuous driving test wasconducted for each of the obtained piezoelectric actuators. Theevaluation results are shown in Table 17.

TABLE 17 A part of a plurality of metals is oppositely disposed bothends of At least one of a piezoelectric layers plurality of metaladjascent to both sides layers is a metal in thickness direction ofMetal parts part composed of metal parts, and the are arranged aplurality of rest are connected interposing in metals scattered throughonly one end between a There are Metal between thereof in thicknessThere are a plurality of Metal parts a plurality composing Samplepiezoelectric direction of plurality of piezoelectric are disposed ofmetal metal No. layers piezoelectric layers metal parts layers regularlyparts parts V-1 ◯ ◯ X X X X Ag, Pd V-2 ◯ X ◯ X X X Ag, Pd V-3 ◯ ◯ ◯ ◯ XX Ag, Pd V-4 ◯ ◯ ◯ ◯ ◯ X Ag, Pd V-5 ◯ ◯ ◯ ◯ ◯ ◯ Ag, Pd V-6 X X X X X X —Voids exist between a Result of continuous driving test plurality ofDisplacement Displacement Noise metal in after generation adjacent tointial continuous of Sample each other in state driving harmonic Beatsound No. metal parts (μm) (μm) Delamination components generation V-1 ◯50.0 49.9 None None None V-2 X 50.0 49.9 None None None V-3 ◯ 55.0 54.9None None None V-4 ◯ 60.0 59.9 None None None V-5 X 60.0 60.0 None NoneNone V-6 X 45.0 42.0 Occurred Occurred Occurred In this table, “◯” meansthat it is satisfied with the condition mentioned above, and “X” meansthat it is unsatisfied with the condition mentioned above.

As shown in Table 17, in Samples Nos. V-6 as a comparative example, thestress exerted on the stacking interface was concentrated at a point,which increased load and caused peeling, as well as beat soundgeneration and noise generation. On the other hand, in Samples Nos. V-1to V-5 as the preferred embodiments of the present invention, noremarkable drop in the element displacement was confirmed even afterbeing continuously driven 1×10⁹ times, and hence these samples hadeffective displacements required for the piezoelectric actuators,respectively. That is, the piezoelectric actuators causing no error andhaving excellent durability were achieved.

Particularly, it can be seen that Sample No. V-3, in which the stressrelaxing layer and the stress concentrating layer were adjacent to eachother with the piezoelectric body in between, were capable of increasingthe element displacement and also manufacturing the multilayer actuatorsexhibiting a stable element displacement. Further, Samples Nos. V-4 andV-5, in which the stress relaxing layers were interposed with thepiezoelectric body in between, were capable of achieving the largestelement displacement, and also manufacturing the piezoelectric actuatorswhich caused little change of the element displacement and had extremelyexcellent durability. This permitted the multilayer actuator exhibitinga stable element displacement.

The invention claimed is:
 1. A multilayer piezoelectric element in whicha plurality of piezoelectric layers and a plurality of metal layers arestacked alternately, wherein a plurality of the metal layers include aplurality of low-filled metal layers having a lower filling rate ofmetal composing the metal layers than oppositely disposed metal layersadjacent to each other in a stacking direction.
 2. The multilayerpiezoelectric element according to claim 1, wherein a plurality of thelow-filled metal layers are disposed by interposing in between aplurality of metal layers other than the low-filled metal layers.
 3. Themultilayer piezoelectric element according to claim 1, wherein aplurality of the low-filled metal layers are disposed regularly in thestacking direction.
 4. The multilayer piezoelectric element according toclaim 1, wherein a plurality of the metal layers includes a plurality ofhigh-filled metal layers having a higher filling rate of metal composingthe metal layers than oppositely disposed metal layers adjacent to eachother in the stacking direction.
 5. The multilayer piezoelectric elementaccording to claim 4, wherein one or a pair of the metal layers adjacentto one of the low-filled metal layers in the stacking direction is thehigh-filled metal layer.
 6. The multilayer piezoelectric elementaccording to claim 4, wherein a pair of the metal layers adjacent to oneof the low-filled metal layers in the stacking direction are thehigh-filled metal layers.
 7. The multilayer piezoelectric elementaccording to claim 4, wherein the high-filled metal layer have a peakmetal filling rate, and the metal filling rate is gradually lowered overat least two metal layers in the stacking direction from the high-filledmetal layer.
 8. The multilayer piezoelectric element according to claim1, wherein the low-filled metal layer is composed of a plurality ofmetal parts spaced apart from each other with voids in between.
 9. Themultilayer piezoelectric element according to claim 1, wherein in aplurality of the metal layers, one having a lower filling rate of metalcomposing the metal layers than oppositely disposed ones adjacent toeach other in the stacking direction is taken as a low-filled metallayer, one having a higher filling rate of metal composing the metallayers than oppositely disposed ones adjacent to each other in thestacking direction is taken as a high-filled metal layer, and a fillingrate ratio (Y1/X1) is in a range of 0.1 to 0.9, where X1 is a fillingrate of metal in other metal layer except for the low-filled metal layerand the high-filled metal layer, and Y1 is a filling rate of metal inthe low-filled metal layer.
 10. The multilayer piezoelectric elementaccording to claim 1, wherein in a plurality of the metal layers, onehaving a lower filling rate of metal composing the metal layers thanoppositely disposed ones adjacent to each other in the stackingdirection is taken as a low-filled metal layer, one having a higherfilling rate of metal composing the metal layers than oppositelydisposed ones adjacent to each other in the stacking direction is takenas a high-filled metal layer, and a filling rate ratio (Z1/X1) is in arange of 1.05 to 2, where X1 is a filling rate of metal in other metallayer except for the low-filled metal layer and the high-filled metallayer, and Z1 is a filling rate of metal in the high-filled metal layer.11. The multilayer piezoelectric element according to claim 1, whereinan inactive layer composed of a piezoelectric body is formed at bothsides in a stacking direction, and a metal layer adjacent to theinactive layer being a low-filled metal layer having a lower metalfilling rate than a metal filling rate in metal layers adjacent to eachother in the stacking direction.
 12. The multilayer piezoelectricelement according to claim 1, wherein the metal layer is composed mainlyof metal selected from elements in groups 8 to 11 of the periodic table,and satisfies the following relationships of: 0<M1<15, 85≦M2<100,M1+M2=100, where M1 (% by mass) is a content of an element in the groups8 to 10 in the periodic table in the metal layer, and M2 (% by mass) isa content of an element in the group 11 in the periodic table.
 13. Themultilayer piezoelectric element according to claim 12, wherein theelement in the groups 8 to 10 of the periodic table in the metal layeris at least one selected from Ni, Pt, Pd, Rh, Ir, Ru and Os, and theelement in the group 11 of the periodic table is at least one selectedfrom Cu, Ag and Au.
 14. The multilayer piezoelectric element accordingto claim 1, wherein the metal layer is composed mainly of Cu.
 15. Aninjector comprising: a container having an injection hole; and amultilayer piezoelectric element according to claim 1, which is housedin the container, the injector being configured so that a liquid filledin the container is discharged from the injection hole by driving of themultilayer piezoelectric element.
 16. The multilayer piezoelectricelement according to claim 1, wherein a plurality of the low-filledmetal layers disposed between the piezoelectric layers are composed of aplurality of metal parts composed of metal and voids, and a plurality ofthe metal parts are spaced apart from each other with the voids inbetween.